First, you figure out what the thing is actually worth. Next, you contact the lender and ask what the payoff is. If the payoff is lets say 1000. dollars and the bike is worth 1000 dollars, the seller gets nothing. (he wont like that)
If the payoff is 1000 and the bike is worth 1500, the lender gets HIS 1000 and the seller gets 500.
Make sure you have possession of the bike before paying it off. (you don't want to pay it off and then find out the seller changed his mind)
If you don't have $ to pay it off, you'll have to apply for a new loan. Good luck.