<HTML>if it doesn't turn over, it could be a few things.
If the bike has been standing still a long time, without a carb on it, or with the carb in an open position, the piston could be rusted onto the cylinder.
The previous owner could have had a .... what's it called in english .... well, the piston could be jammed into the cylinder because of a broken pistonspring.
You could have a broken gearbox (if it has gears, I don't know)
The crankshaft could be broken (I mean in-2-pieces-broken)
Anyway, you should disassemble the cylinderhead, cylinder and piston and (maybe) split the engine. When you close it up again, be sure to use new gaskets.</HTML>