kz750 twin seized engine

I Have a 79 KZ750 twin that i have started to rebuild, The engine that I am working on right now is locked up. I was able to remove the engine and get the top end off to get a look at the pistons after letting it set with pb blaster, marvel mystery, and tranny fluid mix in it. Right cylinder seems to be the problem with it being rusted pretty bad. cylinders are in the down position and now im stuck on what to do next to try and unlock the motor.

Give the PB Blaster a day or so to work. At some point you may have to get a piece of wood and put it on top of the piston and hammer on it to break the piston loose. Needless to say, you are going to either have to bore the cylinder oversize or get a new/used cylinder and piston/rings.