The SD40-2 was the first

The SD40-2 was the first "modern" Blue Box engine. SO it has the yellow flat can motor.

If the motor is in good shape, the comutator cleaned, with good brushes, then you do not need to replace it for DCC. You CAN replace it and get a better running unit, but you don't NEED to.

As for the rear worm drive assembly. you could replace the entire truck with the modern Wired ready to run truck http://www.athearn.com/Products/Default.aspx?ProdID=ATHG63912

If you just need the Worm, Athearn sells that too.

http://www.athearn.com/Products/Default.aspx?ProdID=ATHG40063

They sell the worm assembly as well. Just poke around Their site. You can also find it for sale on ebay and if you are near a Hobby shop, they should have them too.

Since the Prof is probably getting his beauty rest....

I'll quote him:  "The search box is your friend!"

There have been lots of threads on this forum, many with links to YouTube videos, that discuss this very popular topic.  

Indeed, hardwiring an old Athearn for DCC is a rite of passage for many HO modelers.  It is not hard.  I will suggest that you may also need new motor mounts.  Some old Athearn motor mounts stay pliable for decades.  Some turn into peanut butter brittle.  Athearn still sells the old-style white mounts tgat press into the frame.  They also sell the newer design that is held in with screws.  (Don't over tighten the screws!)

As for getting a new worm, Athearn also sells worms but they are designed to work with the newer "dogbone" universal shafts.  Many hobby shops have parts lying around, but hobby shops are getting harder to find.  I'm sure somebody on eBay has some to sell.  The universals come in different lengths, so be careful.  A too-long shaft can be shortened.  A too-shorty, heh, no.

Scott Chatfield

Veteran of hardwiring several hundred Athearns

When you get to this point...

consider a Decoder Buddy and a 21-pin decoder from your favorite manufacturer.  It will leave you in a spot to change your mind at a later time.

It’s not an SD40-2 but it is an Athearn and the parts are essentially the same.
 

Here is a link to an article that describes some of the things to consider with the mechanical parts.

http://www.mcor-nmra.org/Publications/Articles/Athearn_TuneUp.php?fbclid=IwAR1y6_vRCNabtbX8Yp2Ol_E1OntPIdXQjYUGf6n7G8VXGxNRSmzAcf-b_oY

It’s not hard to do and the result is rewarding.  Enjoy!