Unsticking an unused engine........

Tore a motor down a while back and wanted to free it up before to check. I shot some penetrant in each cylinder, it broke free but would only rotate a little, about 20 degrees.
Once i pulled the heads it was apparent it wouldn’t turn all the way over because there was some powdery buildup in the combustion chambers. Cylinders are fine for a rebuild.

I would just try to break it loose and not worry about getting to turn over completely, then tear it down.