One trick I used to "unstick" a few engines that were seized up,was to remove the head,fill it with liquid wrench or similar penetrant,and set it next to my wood stove,and let it soak up the heat for several hours..after awhile it'll get almost too hot to hold onto for long,and they broke free easily..
A few guys I know drop the whole engine and submerge it in a barrel of kerosene or diesel and let it soak for weeks before attempting to free it up..
I put a few in the hot tank at the auto parts store too,that seemed to work well..
The warning about the stuck valve is good advice,I have snapped a cam on two on a rare engine by being too hasty about getting them freed up ,I thought if I rocked it back and forth the valve might break free,instead the cam broke!..
I found one to replace it luckily..
One guy I know tried the grease gun trick to "unseize" a stuck 6S Briggs on an old walk behind tractor--he kept pumping and pumping until he could barely push the handle down...then heard a loud "WHACK"!!..it still wouldn't turn over,so he took the head bolts off..and it came off in two peices!..after wiping off a ton of grease,he saw the top of the piston was caved in like someone bashed it with a ball pein hammer,and it was cracked at the wrist pin..I dont know if the rod was bent,he ended up tossing it in the scrap pile at that point!..lucky for him I had two of those engines in my shed looking for a new home!..