To hold the flywheel and clutch still without the tool (assuming your top end is intact). Take the spark plug out and feed a long piece of rope down the plug hole till you fill up the cylinder. That way the crank cant turn and the nuts can be taken off without everything spinning! You should do this with the piston almost at the top of the cylinder that way you dont need to use as much rope to fill the space and you can be sure it wont get hung up on a port when you pull it out.