OK, I know there are lots of ways to do this but...
I've not been flying for a year or so due to health reasons. I have at least 5-6 models hanging on the garage wall with "castor stuck" engines. Mostly Aero/Thunder Tiger .36's.
I've done the oven bit with Foxes but what's your choice for ball bearing engines with plastic venturis? How much time at what temp in the oven? What else? I don't have a crock pot...
Ward-O

If it's stuck good.. you would have to remove the head, and hopefully the piston is either up top or midway. Heat the engine where the piston sleeve is with a heat gun, then carefully push down on the piston with a deep socket or a rod about the same dia as the piston. This method usually works without damage to other engine parts.. of course remove the plastic venturi, and whatever you do.. don't force it by turning the crank, you might end up with a broken con-rod.