What would be a good DCC decoder for the subject engine?

You'll need to remove the board installed from the factory. A Loksound 5 nano will have to be hard wired in. Loksound has a ge sound file, but it's for a 44 tonner. If you order, you have to have the file loaded into the decoder. Digitax has a N scale sdXn147ps. With this you need a pr3 or pr4 to down load a sound file. Digitrax has one for the  45tonner.

TSU-1100 Tsunami2 is what I used.  Very tight fit for connection wires.

I had a popular vendor replace the Bachmann decoder (+ install a Keep Alive circuit) in both of my Bachmann GE 45 Ton locomotives (see my other post as things did not go smoothly with this vendor). OK, according to the invoice the vendor used a TCS 1425 M1-KAC silent decoder and a TCS 1454 KA1 Keep Alive Capacitor in each locomotive. One of the GEs runs very smoothly whilst the other so-so (some gear noise or "rattling"). I really wanted each locomotive to be sound equipped, but the vendor gave me an inordinate amount of 'push back' so I had to settle for the silent version. Oh, and gone are the dim, orange headlights...nice bright LEDs now.

Ray

Here is a late-breaking update to my Bachmann GE-45T repair/upgrade: I presently own 3 of these 'beauties' and really love how they run on my layout. [I run my trains virtually everyday.] #4 has the original Bachmann decoder to which I added a TCS "Keep Alive" unit. This locomotive runs like an expensive Swiss watch. #5 is currently apart - awaiting my order of micro LEDs. [Check out my previous posts.] A well-known vendor installed a TCS decoder and a TCS Keep Alive unit. Said locomotive is still in pieces as I had to 'chase down' some shorts and will [shortly] replace the inoperative front LED (however there is a light at the end of this tunnel...no pun intended). GE-45T (#6) has been 'dead' for a few years and will garner my attention next. I contacted TCS technical and they advised that I check the motor to see if the non-running begins there. As far as decoders are concerned (for the GE-45T) they recommended: 1) TCS M1 (no Keep Alive...but you can add their miniscule TCS KA4 Keep Alive, or 2) the TCS KAM4-LED which is set up WITH a Keep Alive Circuit and LED wires/resistors already attached. Now the KAM4-LED is a bit larger than the simple M1; BUT if you factor in the additional size of the Keep Alive circuit module you may be better served by going with the TCS KAM4-LED. I'll try to update this post whenever I get to the repair/upgrade of #6.

Cheers,
Ray