no man, you just cannot do anything.
why the hell your friend is creating a problem with this ?
Either you convince him by any means that you need to get this engine on your book or just wait and see when he changes his engine and go to excise office to change the number.
By the way, normally people don't go that soon after a swap to get the engine no. on book, obviously, they surely check it before getting that done for a month or two to make sure the engine is okay.
So, best bet for u is to return it as i am getting from your replies that you cannot force him to do anything