last week. I have had a lot of interruptions, but I finally got the head off the engine. I didn't find any big surprises, the piston was a little scuffed up on the skirt and everything was covered with a lot of stinky black carbon. The scuffed piston is normal for a machine that has been abused and the carbon is a result of all the oil that is being burned.
I think I will just give it a basic top end overhaul: piston, rings, valve seals, head gasket, and hone the cylinder. It should be less than $100 for parts and only a few hours of labour. I still need to investigate why it overheated. I'll check out the electric fan wiring system and temp sensor. I'd be willing to bet it's all working fine and the real problem was that the owner let the kids take the machine out moose hunting. Who knows what teenage boys did to it when they were out in the country?
In the photo you can see my homemade valve spring "tool". It is a piece of 1/2" EMT (electrical conduit) with a cutout on the end. The other end has a block of wood that you can push on. I clamp the head down to the workbench, push on the block of wood with my shoulder, and reach in the cutout on the tubing to remove the spring keepers. There are nice manufactured tools for doing this job, but they all seem to be made for automotive engines and are too big for the small engines that I work on. If anyone knows a good source I'd like to hear about it.