It sounds like your repair guy got himself two free spare motors at your expense (unless he returned them to you?). You sort of answered your own question in that somebody years ago had turned it into a powered unit and it ran fine that way up until you took it in for service. The simple fact that you were running them without issues prior to taking them to him and the fact that he then said they "wouldn't work" even though they obviously had been makes me question his knowledge and abilities as a repairman unless there's more to the story.
The casting part number for both frames is 2333-20. The only difference I'm aware of is a bit more factory machining work on the powered unit frame for the battery compartment opening and the E-unit slot. I don't see how it would make a difference whether somebody added motors or not as long as the direction and speed is staying in sync with the other unit.
