I think I found them on Digikey. The challenge is they are a standard but smaller than normal with a very small wattage rating. I'm going to get both and see if they fit (I have an open and unused chassis just sitting here to allow me to do that!). I'll update with some results once they arrive.
Thanks for the offer @gunrunnerjohn, but since MTH will stop making the engines, I need a plan in place to modify hundreds of engines that I will very specifically NEVER use because I am unlikely to buy new engines anytime for this small layout. Why not, right?