Molasses might work to free it up - if there is rust in the cylinders. If you don't have anything you need the barrel for, it can't hurt. I'd probably remove the flywheel just to get rid of the excess weight. It can go in the barrel along with anything else that needs cleaned.

I've only done the short term disassembly on stuck engines which usually involves breaking the pistons that won't come out naturally. The rings can fuse to the cylinder walls and be a bugger to get out. My process involves force and cans of PBlaster - penetrating oil.