With an adjustable stem, I don't see an issue with getting a 5mm spacer. It won't be that hard to replicate your riding position. It's also not all that hard to cut 3mm off a steerer. If you cut it once, why can't you repeat that effort?

As far as the crown race, assuming you have cartridge bearings, I'd clean up the nicks and run it. Single replacement parts are pretty easy to source for a King headset, so if the bearing dies, you can deal with the race then.

FYI: There are some companies that actually ship headsets with a "split" crown race for cartridge bearing headsets. Makes the install on the fork nearly effortless. I've started to cut my own slots in races for cartridge bearing headsets. I especially do this on newer road forks with a big shelf for the crown race (makes for seamless look when installed on sympathetic frame), otherwise it'd be a pain in the butt (and somewhat of a sketchy removal on a carbon fork) to get the race off to change headsets or remount the for to a different frame.