Hold the rear brake on... which will hold the clutch drum still... might also need a lot of weight on the back of the bike to hold the tyre incontact with the floor for added friction.
doable, but will kill the clutch shaft output shaft and all the support bearings if you keep using this technique.. 50's are especially weak/prone to snapping when you do this. altho it does work.. impact wrench or holding tool... buy them, if you dont want to buy them dont work on your bike. need holding tools/impact wrench and a flywheel remover at bear minimum to work on these engines