BLARG I spoke a little bit too soon. The gasket and crankcase parts are holding fine, no sign of any sort of oil leak, everything seems to be perfectly fine there.
But the starter fork is still acting up. On this bike, the starter gear is engaged to the flywheel by a spring-loaded solenoid that pulls a lever, moving the gear on a shaft. This appears to be sticking; when I try and start the bike, the solenoid doesn't appear to have the power to fully pull the lever. Before I took everything apart, it did slide smoothly, and now it takes a lot of effort, so something is going on.
Right now what I end up having to do is take off the drive gear cover (three allen bolts), and pry the drive lever collar outwards with a screwdriver, to un-stick it. Then I can hit the start button, and the starter engages the flywheel fine, and starts the bike without problems. Of course, it's quite annoying to have to pull out the tool bag every time I need to start the bike.
I've tried spraying some WD-40 on the whole mechanism, which hasn't done much. Right now, my most likely option is having to take the whole crankcase cover back off and muck around with the gears in there; an option I'd rather not do, not just for the PITA factor, but also because I'd have to drain out all of that $5/liter oil that I just put into the bike. Everything's expensive up here :\