I've got a couple old engines that are gummed up pretty good. I can rotate the crankshaft, but there's a lot of resistance from a buildup of old oil/sludge. Is there any one product that's better to spray/soak the engine in to help break some of this stuff down and free it up?

I have heard of this automatic dish washer detergent method before.

Cleaning Model Engines - Click Here

Also I have heard of cooking the engine metal parts in a crock pot with antifreeze.
Be carefull, I reckon don't do it indoors and don't ever use the crock pot for cooking food again.

Or if the dirty stuff is just built up general crud and fuel residue (not cooked on varnish from running fuel with castor oil), attack it with a toothbrush and different types of solvents. Gasoline might work.

After any cleaning, rinse well with hot water, dry as soon as possible and throughly oil, paying particular attention to getting oil into the ball bearings and any other steel parts on engine to keep from rusting.

Remove carb from engine and clean sepperate. Probably rubber O-rings in needle valve assembly that some cleaning agents and/or heat would damage.

Also, it seems that I have seen something at hobby shop just for the engine cleaning purpose.

The Detergent is called Dawn Power Dissolver...............

DO NOT DO THE ANTIFREEZE METHOD. I did it to one of my engines and it was never the same. Not only did it ruin the finish but it never ran good after that. I eventually had to replace it. I don't think there is any easy way to do it.

I had very good luck by just soaking the engine in glow fuel overnight. Everything is free and smooth now.

Now that sounds like a very good idea, wish i had tried that before the antifreeze.