I have done two Stihls they were both very good. The one below with the short hub is FS65AV. I think it is a leftie thread. It does not matter since I used the crank shaft to secure the prop. The hub is basically a washer. The one that looks like a "real" airplane engine, I forget the model. Stihl is very good stuff. For a good price I would grab up most any Stihl. The displacement should be on the tag fixed to the tool. I would not mess with anything under 21cc. Check and see if they have compression. The only thing that difinitively kills these engines is scoring the piston and cylinder. That happens when lubeless gas is run. IF you can't turn them over pay less. Sometimes the oilseals and berings are shot. This is not normally a big deal to replace them......unless the bearing has spun in the race or the crank has run inside the bearing with the bearing siezed. I run about 30% junk when I buy trash tools. 70% is pretty good for junk. I really wish the local dump would allow salvage. They have dozens of old gas tools awaiting destruction.