This will get you started

 

Are the gears etc. that the engine connects to loose (enough) to permit backlash between them and the engine? Just trying to establish that there is enough slack so that if the crank were loose, it is permitted to move just a little. It sounds like it's solid. Can you get a bar into the electric start opening to pry on the ring gear? That's about the maximum leverage you'll get so if that doesn't budge it, you'll probably need to pull the cylinder head after draining the coolant. Then you can pound on the pistons that are at halfway down with a hardwood block and large hammer. Just put some washers and blocks on the cylinder head studs and screw them down to hold the cylinder liners down if you manage to turn the engine over