To be honest, I'd be real surprised if a caseback from another watch fit. And even then, I'd bet dimes to donuts it wouldn't be remotely waterproof. Remember this is the entry-level 1665 you've got here. I'd recommend you just enjoy it for awhile. If you really get the bug to go big with a 1665, your best bet is to pick up a good MBW and scavenge parts from this watch to build up the MBW (datewheel, movement, insert). Then you can start with the gonzo upgrades. But like Pannie153 said, it's easy to go waaaaaaaay too far. Enjoy what you have for awhile.
A hint learned the hard way: if you decide to go big with an MBW, let this be a lesson. Don't settle for something that's "almost good enough" when you really have the bug. In the end you'll buy the one that really IS good enough, and all the money spent in the interim is wasted. Save for the big kahuna, then jump in with both feet.