4 years ago my boyfriend bought me a scooter on payments. It was put in my shed and left there, for 4 years now! No one came to repossess it and as far as I know he never paid for it. I haven't been in that shed for at least 4 years and he and I haven't spoken and I don't even know where he lives! Can I put this in my name and use it? It has been here for a little over 4 years now.

If it is registered in his name, then he would have to sign over title to you before you can register it.

If Banks relied on Memory, you might be OK, but they have Computers to remind them what is theirs. They would still have the Title.
Even if the bank let you pay the Balance, you would likely have to get his OK in writing.
Hopefully a better answer will follow.
Good Luck

Stratmando, there is no better answer. There's a lien on the bike. Maybe they can't find him but once she tries to register it they sure as heck will find the bike!