They seem to be prone to blowing head gaskets Aldo, but that is a relatively minor problem because it is easy to diagnose and easy to fix. They also seem to wear out their crankcase-pulse fuel pumps, which again is easy to diagnose and fix. There are more important problems which are not so easy.
First and foremost, the decompressor wears down so it stops working and the starter burns out trying to start the engine on full compression. There is no way to avoid this except to keep adjusting the exhaust tappet clearance at least once a year, until you find you have had to reduce the clearance below the nominal setting to get the decompressor to work at all. Then you have to replace the whole camshaft, because the cheap piece-of-tin-on-its-edge that is the decompressor, is not replaceable. Most people seem to fail to diagnose the problem at various points along the way, and consequently keep replacing starter motors, wiring, and batteries, which can be quite a burden.
The other serious problem only applies to the twin cylinder version: until the last couple of years, it had a badly designed air cleaner box which was very prone to dusting the engine.
If the engine you are looking at has run 500 hours without its camshaft being replaced, it is most likely now in need of that operation. You can probably tell by seeing whether the decompressor is working (if the engine will crank properly, it is working), and if it is working, check the exhaust tappet clearance. If it is already less than the nominal setting of 0.005-0.007", it may be because it needs a new camshaft and somebody has been trying to keep it running.
The other thing I'd suggest, is that if it is a twin cylinder, don't buy it. There is a completely redesigned later air cleaner box that works properly, but I think it may be quite a hassle to convert an old engine to use it.
