You can drop the whole engine in. Depending on what you're trying to accomplish, you may do different levels of disassembly.
For a frozen-up engine that you are going to clean up, restore, and try to get working, take off anything that you can, especially the backplate and the head. Plan on new gaskets everywhere.
For an engine that you know ran, but was put away years ago without flushing it, and now you can hardly turn the crank, you might leave the head on (remove the plug) and cook for a day or two. It should free up unless the inside was really varnished up and/or has now rusted. If it does free up, then flush it out, and oil it well and give it a try. If it was an acquired engine, I might pull the backplate and check for rust. (If it's a second hand RC engine, I always do this. It probably ran without any castor, and if its hard to turn its not from solidified castor, so....) The crank is usually a good indicator of what is going on inside. If I see rust, then I pull apart the whole thing to see what else is bad. Running loose oxides thru your engine on its first start just grinds away at the fits.
For an engine that you are running, but has a case of the crustys on the outside, same as above, but without the likely unpleasant "discoveries."
Most of the engineering plastics that you find on an engine will not be affected by the antifreeze, and not by a low/moderate temperature. If your crockpot has a low setting, use that.
Don't leave the pot in an enclosed space like a garage or.... Some of the heated antifreeze vaporizes and deposits on things cold enough to cause condensation. (ie. Everywhere. Including on your unpainted 20-pointer hanging on the wall waiting for good weather.)
Antifreeze will remove paint, so don't dunk your red heads, green heads, or blue heads. Unless you want a natural finish engine or plan on repainting your head as part of your restoration. I have not cooked an OS LA blooie, so I can't tell you how that turns out. I imagine that it doesn't all just come off and leave you with a sparkling silver engine. Nothing is that easy....
It may discolor some aluminum castings. Take it from a fairly bright original finish to a mottled, darker finish. This has to do with something in some of the older aluminum cast alloys. I've seen it on several old OS FSR cases, for example.
Dave