Got a little time yesterday to work on the motorcycle. I still need to gut the carburettors (rebuild kits came in the mail a couple weeks ago), but I got a chance to compression-test all four cylinders. I checked each spark plug for a spark, gapped each one, and while the spark plug was removed compression-tested each cylinder. Cylinder 1 stroked at 120psi but the other three did 113psi. That's still within the 10% tolerance the service manual dictates, so I think this engine's okay. At the very minimum, I can get this engine running and take another compression test when it's warm; I don't know if I fully trust a cold compression test.
That the spark plugs all fire properly is another indication that the bike's troubles are carburettor-related, which doesn't shock me. When I first got it there was 20-year-old gasoline still in the tank, which doubtlessly had made its way into all the fuel valves and carburettors over the years. I think I may insert a small fuel filter inline with the fuel line that goes from the reserve valve to the carburettor assembly. At least for the first few thousand miles of riding it should have one, just to make sure whatever's left inside the fuel tank doesn't gum up the engine.
Alright well I'm back to work, space cadets; more updates later if I get those carburettors out.