DalaGStanator's Customs, Mods and Experiments

Sounds like you're going to be very busy indeed, I agree the GT3 could well be a good fit for a Spencer chassis, but would probably work just as well on a Gordon or James depending upon what you have to hand.

One small engine you may want to consider for the small wind up chassis would have to be the ultimate original steamer - the Rocket, I think that would look great on a small chassis.  Unfortunately I missed the release of the new Hornby version during the pandemic or else I would have definitely added that to my collection.  Talk about a miracle of miniaturisation.  I thought the little Rushton was stunningly good, but the Rocket does have far more detail due to its nature.  The recently modelled Hornby Peckett's are also nice ones to look at, very much like a more sophisticated version of Bill and Ben.

I'll have to have a look at the activator tab/motor retainer to see if I could replicate some on my 3D printer for you, which chassis design ones are you looking at?