Today, I finally put on the rigid fork I'd bought to replace the old RST suspension on my Dawes MTB, with Aheadset, having assembled enough spacers to get around cutting the steerer. I tightened up the topcap bolt first, then the stem binder bolts, but when I'd finished, I noticed there was lots of play in the headset. I then saw a little ring of black plastic that had slipped out around the bottom of the headset, which broke off fairly easily when I pulled at it.
Looking at Sheldon Brown's headset page, there doesn't seem to normally be a plastic ring there, but on the Park website, it looks as if there should be a fork crown race, which seems to be missing from my current assembly--the bike's bearing retainer is at the bottom when I loosen everything up to check.
I can't figure out how this would be missing, though, unless the thin, broken plastic ring was the original fork crown race, or if the original one was lodged on the former fork (see photo). I tried to loosen what looks like possibly the fork crown race from the old suspension fork, but it shows no signs of budging.
Any advice about this would be a big help. Thanks. (First photo is new fork with plastic ring showing; second photo is old fork, with seemingly integrated, solid fork crown race, but please let me know if I'm wrong.

New fork has not had a crown race installed.

The old fork has a crown race installed. It is re-useable IF you get it off without damaging it, AND the new fork and the old fork have the same diameter seat for the crown race.

the plastic ring is a dust ring to keep dirt and whatever out of the headset . to remove the crown race , you need a puller or an punch and a hammer and give it good hard blows as you work around the race . It should come off with few good blows .

Ss noted, when you switched forks, you never transferred the crown race from old to new. It seems as though you ended up wrecking the weather seal, though maybe not. However you need to transfer the crown race, then assemble correctly with or without the seal.

ince the race seems to overhang the crown, removing it should be easy. Set some sort of punch under one side and tap it upward. Then switch sides and working back and forth work the race upward keeping it squarish until it's free. Mounting is done with a piece of pipe slipped over the steerer and driving it down.

Thanks a lot for all these replies. I'll give it a shot removing the crown race from the old fork. If this works, I'm wondering about replacing the plastic piece--I'm not seeing replacement headset weather seals, or gaskets online. Is there a different term for this?

To my knowledge, these are rarely, if ever, sold as spares. Depending on the headset, you might get one by emailing the maker, but most likely you'll need to do without. The lower seal is more important than the upper, so if possible switch the other one down. Then improvise a seal with an O-ring, if you can. Or you can fit an exterior seal with a wide rubber band cut from an inner tube or using the one they use to hold broccoli together.

Thank you. I like all this advice, and I've learned a good bit about the headset through this process. I did manage to get the old crown race off the fork, and it should be usable. The parts were pretty mucky inside when I took the headset apart, so I'm going to check it over and make sure the bearings are OK before putting it all back.
Would you expect a normal plumbing o-ring to fit around the crown race? Otherwise, I'll try some innertube on the outside (you don't see those broccoli rubber bands much in the UK, though I know just what you mean).

O-rings come in an almost infinite number of diameters and thicknesses. Try to gauge the gap, and look for a ring that is slightly bigger than the ID of the "skirt" on the headset, so it'll stay up there, and thick enough to sit above the outer corner of the crown. Otherwise the cut inner tube will be your best bet Also depending on how the crown recesses into the skirt of the headset (varies by brand) You might cut a donut from a sheet of plastic, and trap it under the crown, to support an O-ring above it.

I improvise headset seals as needed, but they're all different depending on the gap and recess between the cup and crown race.

Thanks. I'll look for an O-ring today to finish off the job. Could you explain the fitting in relation to this diagram? It's my first encounter with the insides of a headset, and I think I understand the "skirt" is what they're calling the "cup" here, but not totally sure. Thanks again:
Bicycles/Maintenance and Repair/Headsets/Headset Overhaul - Wikibooks, open books for an open world

Skirt is my word, meaning that section that hangs down shielding a lower area, like a woman's skirt. Take a look at the cross section sketch here. By skirt I'm referring to that thin vertical area hanging below the bearing area on the cup and extending down to overlap the crown race. Your headset is a bit different in that there's a shield inserted to narrow the gap between the skirt and the crown race, but the concept is the same.

You need to improvise an alternative, that will close that gap, staying below the bearing area, yet not falling down below the crown race. How you do that depends on the exact design, and where you can trap the O-ring so it stays put.

On some headsets, the easiest way to do this is to trap a disc below and extending out from the crown race by 1/8" or so. Then an O-ring can rest on that, and the shirt can either meet it end on, or hang down to close to the shelf trapping it.

As I said earlier, every headset is different, so the exact solution will call for some improvisation.

Went looking for an O-ring, but failed because of the hardware store owner's unusual attitude (see: corson the rules). I aimed to shop locally, but will probably have to resort to a bigger retailer for this.

Thank you to all who replied with great advice. I finished the job yesterday, with a weather seal improvised from two o-rings, as FBinNY advised, and took it out for a test ride along the canal path. While I had it all taken apart, I switched the top and bottom headset bearings, cleaned and regreased with lithium grease. All set for now--next step, full mudguards, if possible.