I completed a Chivers Hunslet 0-4-0ST (Dolbadarn) with Arnold 0-4-0 chassis and it appeared fine initially relying on friction to hold the rear end of the chassis in position (there is a clip at the front) but under load the worm
Frankly, I would glue the motor in place with epoxy
Another suggestion would be to use rubber sealant in place of the epoxy because at least it mens that you can get the motor out of the chassis in future by cutting the sealant with a scalpel . Silicone is fairly easy to get but there are others out there on the shelves of your local B&Q or other DIY 'supershed', including PVA and uretane types which will not be so harsh chemically. Whatever you use though just let it set without the body on with the chassis sat on the workbench so that anything the sealant produces as it goes off (i.e. acetic acid from the silicone sealant) is not trapped around the important parts of the chassis like the motor as corrocion can be a b****r - as pilotfriend can tell you from experience! ;).