Pulley and belt alignment problems?

You could use the rope trick.

Bring a cyl to TDC of the compression stroke, back off a bit. Stuff some cotton rope into the cyl through the spark plugs hole, rotate crank till the rope gets compressed and you can not turn the crank.

Remember you will need to do this in the right crank turning direction to remove and again to reinstall to work.

Be SURE both valves are closed or you could bend one.