After tapping on the case joint for what felt like an hour I was able to wiggle the halves apart and gain access to the engine internals. If there's a simpler multi-speed motorcycle engine, I would be very surprised. That balance shaft is the only unexpected component, and I suspect the engineers put it in there to complement the inertia of the crankshaft, not cancel vibrations. A few minutes in the oven persuaded the balance shaft out of the case so that the offending bearing could be removed. It's a common size, so hopefully I'll be able to pick up a replacement locally. I then proceeded to inspect the rest of the gearbox. Out came the caliper and feeler gauges. Everything is well within specs, but the cylinder has some scrapes from a seizure and should be replaced (or as the Haynes authors would say, renewed).