This is the golden age for kit bashing opportunity: thinking back to the 60s and 70s, just a quarter of the present RTR selection would at that time have left us weeping with gratitude at what this made relatively easily possible.
I am a timetable operator above all else, and the advent of reliable smooth running mechanisms from late 1990s onwards Chinese production, has enabled a good number of mechanism transplants into both kit and RTR bodies, some mine and some for friends. To date I have utilised the following mechanisms:
Bachmann: 56xx, 57xx, Jinty, J11, K3, V1, BR 5MT, TOPS 20, 24, 45, 55
Heljan: class 27, 128
Hornby: A3, K1, 8F, M7, 7P, BR 7MT, 9F, class 30
Oxford: Adams Radial
The other side to this is that almost as fast as I complete a mechanism transplant into a kit or a (usually modified) RTR body, this proves to be the 'rain dance' that causes announcement of a 'current standard' RTR model of the class concerned. Those of mine 'superseded' (or about to be) include Pepp A2, B17, J50, N7, Thompson O1, V2 (twice!) DP1 TOPS 21, 23, 30, 40. For this reason I have been hesitating on an O4/7 from a Bachmann O4 and old Replica B1 body ...
My C12, J6, J39, J52, N2, look likely to put in a few more years yet, while the 'neverwazzas', Gresley G1, P3 2-8-2T and Pepp P4 2-8-2, are sure of long life.