I bought a 72 K2 750 in June.The P.O. had said that it hadn't been running in a while and had started to tear it down.He gave up and bought a new bike when he couldn't get the motor out of the frame...he hadn't pulled the oil filter housing

I've got the bike completely apart now and have had the cylinders soaking in every kind of solution I and others on this forum have suggested.
I've dutifully given the kick start a little nudge every day but no luck.If I shift the motor into neutral the front sprocket spins freely.The valves are all ok and the cam chain is fine.
Now I need some advice,I have no qualms about pulling the head and opening the cases but I want to try everything I can to unstick the pistons first.There are 2 things that could be toast that I can think of:
1. The primary chain is munched
2. The tranny is shot
If these are the problem then I open the motor but if anyone has any suggestions I would really love to hear them.
Thanks