My suggestion would be to contact Bachmann's parts / service department and inquire of buying a "DCC ready" tender for your engine. Bachmann has a pretty good supply of parts, and they aren't all that expensive. You just unplug the harness from the tender to the engine, and plug in your new DC tender. If they don't have a Maine Central tender, you can just get any tender that's correct for that engine (any other railroad, or undec) and swap the tender bodies.
You could then sell the sound decoder equipped tender, probably for twice what you paid for the DCC-ready tender. I believe the 2-8-2 uses the same USRA tender as several other Bachmann engines, so I'd bet a lot of folks would be interested in buying the sound tender.
https://www.bachmanntrains.com/home-usa/index.php