A physical inspection should be first before ordering anything.
Sadly, Lionel doesn't sell the 691RCVRB01 separately anymore, and if anything that would be the problem board. The 692RCMC430 is the complete RCMC and TMCC receiver assembly, likely overkill to fix this problem, but it may be the only game in town.
I'd first check and see if the 691RCVRB01 board has simply become dislodged from the RCMC main board, those earlier RCMC equipped engines didn't solder them together. Also, I've had a couple that had a broken pin, easily repaired, and all was well. Also, make sure the antenna plug that goes to the little TMCC receiver is connected and the antenna isn't shorted to frame ground.