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Bachmann may have spare parts that you can purchase to make your DDA40X serviceable again. 

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Dear ???

Hmmm, DD40's are not a loco I'm usually across, but a quick google suggests:

- Parts are available, athough these may be for a newer "DCC Ready" production run
https://estore.bachmanntrains.com/index.php?main_page=index&cPath=68_595&zenid=g8me1tq00042morvt9b6s43ga0

- New B'mann DD40X has an MRSP over USD$200, so getting replacement parts may well cheaper, assuming you have the motivation and skills to "make them work"
https://shop.bachmanntrains.com/index.php?main_page=index&cPath=258_288_291

- Alternatively, I can see Athearn DD40X mechs
("Blue Box" INSIDE-frame, Gold-side semi-can-motor era)
available for as little as USD$30 on eBay, and complete running locos for USD$90,

and it's generally acknowledged that the agriculturally-rugged Ath diesel mech will outlast/out-perform most other "toy train class" mechs of the 80s and 90s...

(and tuning/cleaning/optimising them, even for the latest DCC installation applications,
is rather easy and dare-I-say, enjoyable...
...use the Search Box at top right with terms "Athearn Mech Tune" for all the discussion and tips...)

Happy Modelling,
Aim to Improve,
Prof Klyzlr

....

This DD-40x may be a good clean slate for you to practice various techniques and skill upon to include sand blasting, spray paint, air brush, weather, etc.  And you could tear it down and take it apart with literally no fear of messing it up more than it already is.  If it's a truly older Bachmann, it's a relic mostly headed for retirement, if not due to the age, but rather how much is broken under the hood.

You could further use the parts for projects including using the powered truck for a home brew electric locomotive of the 4-8-4 (with two freight trucks with pickups) .

If you are seriously interested in having a DD40X and have the range to run it, Athearn has re-issued their beast and it will be a lot further ahead of anything most of us will ever muster in our lifetime.  Their old powered model is still a better starting place than the old Bachmann models.

Sometimes the most important

Sometimes the most important (and often hardest and costliest to learn) lesson is when to abandon a project as not worth the time/ effort or all but impossible to do

-Doug M

and...

From the sound of it, it may be a viable middle ground to scrap the power train and convert this into a dummy unit. I've just done the same thing, although this one was a brass (!) model. As you'd guess from a brass model, the detail is very nice. The paint job, like yours, was in terrible condition. There were bits of three different previous paint jobs on the body, often in places where it was very hard to evacuate. The trucks, though, still had their original factory gold varnish! (facepalms...) After I got done sand blasting it all off, fixing a couple of damaged details, priming, painting and decaling it, I discovered that the gear train is almost as loud as the speaker that was going to go into the unit! Bringing it up to standard would have required repowering, regearing and rebuilding the trucks - more or less what you will need to do. I'll spare the details but the model now has no gears and is nicely free rolling. With some weathering it will make a nice companion to the brand new Atlas DCC/sound unit that just came in...