Regarding the speed of the Clayton, like one or two others here I re-motored my Clayton [using a Mashima 1626 motor]. I now have a loco that runs and pulls well, but appears no faster than it was with the original motor [which is fine by me!].
The motor runs nice and cool, but I did check out the gearboxes as a precaution, Mine seemed fine as regards input shaft end float, but I did ream out the free end bush as there appeared to be a little misalignement, though I don't think it was really required.
Jerry