you can
A. heat it in your oven at 300 degrees for about 15 minuets, and remove it wearing good gloves to remove the piston and sleeve.
B. try taking the glow plug out and putting in some break free, available in the gun department at walmart usually, or some PB blaster, which is available at most auto parts stores, let it soak for a while then try to turn her over.
C. really really hot water, but youll have to make sure you dissasymble it all the way and clean it thouroughly.

all these methods require a new piston and sleeve.

however your first step would be to detirmine whether the front bearing is the stuck part, or if its the piston and sleeve. needless to say, youve got some work ahead of you.

let us know how it goes, most everyone on here is willing to help.