As kobaskle says, if it is seized pour a good quantity of penetrating fluid such as plus gas in, and leave it to work dont rush it!
Seizures are usually piston to barrel on these bikes. Question is why did it seize, check the cable from the throttle to the autolube pump pulley has not broken. After you have left the penetrating oil to work, select a high gear 4th or 5th and gently rock the bike back and forth with the plug removed, remembering to cross your fingers as you do so