Registration on Bike Index is not proof of ownership. I could steal any bike, and register it as my own on Bike Index, but I would not have any proof of ownership. Anyone who claims that registration on Bike Index is proof of ownership is lying.
Can you supply any proof of ownership? A bank or credit card statement, or a reputable bike shop that is still in business which kept a record of the transaction, may be acceptable to law enforcement.
Here with the Eugene Police Department, which has its own closed bike registration system, they will not release a bike to the owner that registered the bike in their own system unless they also provide proof of ownership.