Andrew,
No1 – I would say Maisie was a pretty common design for people to build "back in the day" and there must be quite a few of them around, ours was quite a small club back in the 70's and there were at least three Maisie's among our members back then! Given the one you seek would have needed a new boiler or at least a significant repair which is quite invasive I would think if it was done it would have been at least repainted and very likely to look significantly different now. So finding this one may be the most difficult.
No2 – I don't know what design that may have been but it sounds a little more obscure and if so give there are probably fewer of them around, that may be easier to track down than No1.
No3 – Back in the 70's 7 1/4" gauge was less popular than the smaller gauges and even today 7 1/4" is less popular than 5" – needs more space for a track, fewer clubs have tracks of that gauge, probably still true today. Something the size of a 7 1/4 Royal Scot is not an inconsiderable "lump" to move around. Out of the three this one may be the easiest to track down and given the size of the boiler it would have required would not be something that was built in the average back garden with a 2 pint paraffin blow-lamp! So as a starting point this one may be the one to concentrate on first. A general letter of explanation reproduced to all the organisation's with 7 1/4" tracks might generate a response, maybe also to the 7 1/4" Society? Something this size is less likely to have ended up stuffed and mounted on someone's desk, coffee table or bookshelf!
Good luck with your quest, certainly be interesting to hear how you get on. If you have any photo's of any of them that may help.
Best regards,
Paul.