GP35

The quick reply is it should fit, the key word is "should".  Find a replacement donor for the mechanism and if it does not fit, then just return the silly thing. Does not sound like too bad of a worst case scenario. Have fun with your project. M.

Diagnosis?

Dear Otter79,

GIven that there have not been a whole lot of physical changes (that I'm aware of) to the body shells of the B'mann GP-series locomotive models over the last couple of decades or so,
I would think that a "like for like" body<> mechanism swap would be reasonably plug-n-play simple.

(According to the typically trustworthy HOSeeker.net HO model train documentation archive,
the last major body/mech desgn change was in the 1990s?)

That said, I'm intrigued, how has the loco "NEVER worked"? Model trains are relatively simple devices in both mechanical and electrical terms. It may well be possible to diagnosis and resolve the issue in-house with not-much-more than a small screwdriver, patience, and careful observation.

Indeed, if you're considering performing a body<> mech swap solution yourself,
then you're already looking at doing the same ammount of basic "dis-assembly/re-assembly" required to diagnose and likely-resolve the issue...

If you and/or your husband are soo inclined, I'm sure the gathered luminaries here could help you work thru the basic disassembly and diagnostic process.

Pictures of the actual loco-in-question always help,
there is a "how to post pictures on MRH" walk-thru available HERE
https://model-railroad-hobbyist.com/help/how-to-post-an-image

Whichever way you choose to proceed, wishing you best of luck!

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Don’t expect

Anything but a Bachmann plus drive to fit.  Bachmann is notorious for internal changes, even between runs of the same model.  Fortunately there are several available on ebay for under $20.  
like the Prof says, the fact that it never worked probably implies something simple.  This is one of what I like to call their no wire models.  electrical circuit is made through spring contacts with the two insulated Metal weights that fill the shell.  There are two contact points on either ide if the circuit.  If the contact is lost at any of these the engine won’t run.  It was a problem for them and did not last long.  It is not a DCC friendly design.

Good luck with it.