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).