Ned Simmons wrote:

Now that you've ruled that out, call FedEx and tell them
it's not your engine and they need to come and get it and
do their best to get it to the rightful owner. It might irk
you that the engine may never get to its proper destination
(it would me), but I don't think you have any claim on it.
It's not like you found it in the street with no idea of
where it's been - it was consigned to FedEx and it's their
responsibility.

I would consider this option if I didn't want to spend the time
searching for the rightful owner. But searching could be interesting -
I'm sure it is possible to find the truth if you are really determined.
The truth is out there :-). Maybe some biker guy saw your website and
you became his personal hero, so he decided to send you his old bike
engine? Maybe it really was meant for you? It is rather odd that there's
no documentation inside. Most companies will include an invoice, and I
do the same when I sell stuff on eBay for exactly this reason - if the
label gets lost the destination is clear as soon as the package is opened.

Chris