I must admit that I said yes to more of these than otherwise, I guess I'm a sucker for odd locos. Would 3ft gauge prototypes be 4mm scale on 12mm track or some other scale, is this an established scale?
I would definitley love an Aveling and Porter loco but think one of the 0-4-0 with a cab style engines would be more achievable than the open 2-2-0 "Blue Circle" type. Would have to have the fly wheel spin though, I wonder if you could have the motor worm drive the crankshaft and have a functioning gear train.