Many years ago I acquired a 540c and 520c in a single eBay auction.  Both operate perfectly (the 520c screen was kind of janky), but the 540c's case was slightly beat up/required tape to keep parts on it.  So recently, I cannibalized the 520c's case and moved over the necessary parts to the 540c.  I am now left with a perfectly functional 520c motherboard, processor, floppy drive, etc.

Once the processor is removed, is the 520 motherboard identical to a 540 motherboard?  I am wondering if it makes sense to keep the 520 motherboard as a "backup" should anything happen to the 540 motherboard or if I should try to trade the 520 board.

Thanks!