you can make a bike specific puller adapter for one of those auto steering wheel puller rather easily. 1/4 inch plate steel atleast as big as your magneto cover gasket, using your gasket mark and drill the holes and then drill 1 big hole in the center where the crank shaft is. using long 6mm bolts sandwich the "steering wheel" puller resting on the crank with the plate bolted to your cover mounts. Make sure everything is straight ,even and centered so when you tighten the puller screw it pushes on your plate nicely. Snug up , light tap with a rubber mallet on the countershaft,snug ,tap snug, tap etc.
If you plan on keeping your bike forever or working on a lot of bikes that have the same magneto cover , it is not a bad thing to have for the cost and effort to make .