Not trying to talk down to any one but many times a fella doesn't know clutches are LEFT HAND threads. So now your clutch hub is SUPER tight.
Use a ROPE in the plug hole not a piston stop. Use plenty but don't let the rope into the exhaust or intake port. (pull the starter rope in the recoil out a tad for recoil protection) Take up the Slack at the hub going counter clockwise POT the umph to it with a gadget I neglected to tell ya to make before You did all of the above (SORRY about that). Do your best not to put any cattywampus (Spell check accepts thet word WOW) Pressure and bugger the end of your crank. If you could fabricate a spanner wrench to use those 2 holes in the hub get it already and wrap a cold REAL COLD wet rag under the hub to protect the seals and stuff from heat.....Put a dab of quick heat (Torch) on the hub but not the end of the crank AND LEFTY LOOSEY IT .. if it does or does not work do your best to keep your crank cool. Your spanner pins needs to go into the holes all the way and the face of metal your pins are in needs to be as close to the hub as possible (don't round/smear those holes}
Or use a cut off wheel and cut as safely close as you can to the crank threads on 2 sides. I would prefer option #1 Good Luck.