Even though the bike is in your possession doesn't make it yours - it will be the credit unions until it's paid out.
If it is an option and checks out, can he transfer the debt to you? Then you could pay it out.. I am not sure how it works on your side of the pond..
If this guy doesn't do the right thing, you'll be out the $ and the bike if you're not careful..!!
IMO. It's not worth it. There are heaps of other bikes out there..
My $0.02 AUD