Pretty much found you can get away with dropping the wheel size by about 2 scale inches (down to 6'6 from 6'8) but you get a lot of slip after that. Sonic models have since announced their Glen class, which may be a better option as a chassis for the MGWR A (higher boiler pitch, smaller drive wheels).
Past a certain point you're likely better off scratchbuilding, or perhaps 3d printing a chassis. At that stage might as well do the whole loco.
I have seen a conversion of a dean goods to a 2-4-0 before, but the driving wheels are much smaller than the Irish 2-4-0 survivors. the GWR 0-4-2 tank likewise has much smaller driving wheels.