We ran (and passed) all the tests for the cruise control, so then tightened up the slack in the cable. It didn't help. I played with the settings, and went for a few test drives but haven't changed anything, so I've decided to leave it for the moment.
I spoke to Vince in the evening, and the Flightstar's all back together - I just need to go and add gas and go flying. I was confused at first, as I was sure there was about half a tank in the plane, but he's used that all up running it in. It seems the problem was the crank seals, and the mark I was looking at on the barrel wasn't a seizure, rather the piston ring gap marking the wall. Joy.