A blown head-gasket on an air-cooled single-cylinder 2-stroke is the easiest head-gasket to diagnose. You only have to check for one thing:
Is there oil/gas leaking out of where the head meets the cylinder?
Yes? It's blown.
No? It's not.
That's it. It's not like on a car where it can leak into an oil galley, water passage or into another cylinder. The head gasket only seals one thing: the cylinder. Besides, you'd have to have one **** of a leak for it to really affect performance. I'm talking about seeing puffs of smoke spitting out of the side of the engine.