Ive gotten a few Bikes and sleds goin that were siezed but some are too far gone..
The 69 CL 450 (weatherlocked) Ran Lost spark on 1 cyl
71 Skidoo TNT 340 single (Weatherlocked) Runs and goes
72 Chaprrall 340 (Weather locked) Ran But has bad bearing
Zundapp CS50 (Water Vapor locked Badly) Crank+topend Junk.
74 Canam TNT 175 (weather locked) Needs clutch cable and assorted parts
What i do is take the cylnder head off
Then put some of your faverote oil in the cylnder
Then Get an old wood hammer handle
Hit the handle with a rubber malett
If the engine dosent come free get another set of hands with a breaker arm and wiggle the flywheel nut and it should come undone.
DO NOT USE A SCREWDRIVER FOR THIS!!!!
YOU WILL DESTROY THE PISTON!!!
Sometimes the breaker arm is all you need
Like with the 450 i just had to turn the engine by the crank nut and it came free.
And if the airbox or carb is not on the machene and-or has a open exaust and is siezed good chance the engine is toast.
Aslo beware of No oil siezures. On a 4stroke check the oil when you get the Bike to see if there is any oil in the engine if there isnt good chance the engine is wrecked.