And lapping with some valve grinding compound to start. I ran the lathe extremely low speed, with a cloth over the ways to protect them. You can also do it in the drill press with less worry, but mine doesn't go slow enough not to worry about a catch.
The bore was eventually cleaned up back to round, with about a thousandth taken off. I reassembled the engine and there was still good compression, so I gave it a run this evening. It struggled to break 900 rpm, and at that speed I could hear some pretty loud clacking sounds.
Upon disassembly and checking parts, these noises turned out to be piston slap. The piston is definitely undersized now despite the good compression, and I could feel a fair amount of slap-stick when I exercised it by hand. Not surprising the engine couldn't run well.
As I figured, time for a new piston.
Unfortunately a couple hour search for the stick of graphite I'd had eighteen years ago when first building the engine turned up nothing. I know I still have it somewhere. Extremely irritating! But finally I gave up. I sent for another which will take about a week to get here. I know it will turn up again, some time after the new one arrives.
Never mind, I'll think about making other improvements in the mean time.