Gotta admire your pricing analysis!
I'm not sure how much you're interested in the how-it-works aspect of your repair but Lad raises a good point about transistors and motor/relay circuits. Obviously I can't speak to what Lionel or whoever designed this was thinking but the reasoning behind why a protection diode is not needed here is the transistor does not "quickly" switch on/off which is what causes spikes. The combination of the 75K resistor and the 1mf capacitor form a filter that slowly (in electrical time) turns the motor on and off.
Likewise, I see you're going with a polarized 1mf cap (as Lionel did). My concern was the so-called "reverse bias" on a polarized capacitor if the Bell button is pressed. That is, for a polarized cap you want to keep the + lead more positive than the - lead. But, a Bell button is infrequently pressed and certainly not held down. Plus, after you changed from 7.5K to 75K, this lowers any current that flows through the cap by 10x so I guess I have to look the other way.