Went home, got the unbroken bolt, and went to the local True Value, and they had them: Stainless steel and only $0.50. I bought two. The flange turned out, upon closer examination, to be a bit larger, but this turned out not to matter; it may even be better.
Installed the cam chain tensioner and turned the engine a couple times by hand to make it do it's thing. Re-measured all the valve clearences. The exhaust valves are particularly difficult to measure, since they are on the far side of the engine, and there's stuff that gets in the way. In the end, I had to bend my feeler gauge to get the right angle. I have some tappet feeler gauges which are pre-bent, but they don't have enough precision for this bike; they are probably intended for something larger. Everything checks out within specs.
A thunderstorm blows in, so I'm calling it quits for the night. I'm taking the day off tomorrow to finish up. I still need to get some motor oil. With any luck at all, I should have it running tomorrow afternoon. Then I need to change the oil and filter, and the coolant.