Just wanted to follow up on this. I have not cracked any codes from Lionel for their part numbers but I did get that RS3 back up and running. Holy nostalgia! It's got a basic can motor with no flywheel. The whole engine is very light. It makes a heck of a noise, especially when I hit the horn. It was fun going back to basics and driving conventional. Here's a quick video:
Now...I gotta figure out which route I want to pursue for command control! TMCC, LC+2.0 (if I can get boards), or Legacy (probably not but maybe if I can get boards). Thanks for the help finding parts!!