The TMCC sound drop-out is easy. YLB - RailSounds Battery Replacement, we have them for TMCC with standard battery clips and for Legacy with no battery cable.
How about a picture of the underside of the problem MTH tender trucks. Many of them have the mounting provisions for standard MTH hardware. If they don't, that can be accommodated, just a bit harder.
MTH warranty won't cover modifications like this, so that's probably a dead end.