Update: the broken bolt defeated all attempts at extraction, so after a long time remounting with the "universal" mounting block from bikeberry (which I had to flatten and hammer one of the brackets to fit because it doesn't even fit its own mounting block) and adjusting the wheel and sprockets so the chain doesn't fall off, and replacing the old fuel line (which had earlier fallen off while testing and sprayed me with gas), I took it out in the alley behind my house and...
...the engine runs but the clutch won't engage.