Yeah you might be right, could have been something totally different. Even a good machinist can mess up I guess. He did show me the valve after he took it out though, and it had what looked like dark oil all over it, except it was a much thicker consistency... I guess the question is whether it actually had anything to do with the compression problem or not.
Either way, it's all back together now and I checked compression across all cylinders and it looked good!
Then I rebuilt the carb, hooked the fuel pump up to a can of new gas, turned the key, and... nothing. Couldn't get it to even try to start. Not even with some starter fluid sprayed in. Will take a step back and check everything over tomorrow night, I think I have an ignition problem somewhere.