I think most of the later engine covers are very close in dimension, but the ST and the Colt had the sides cut away to sit flat on the ridge around the engine compartment. I'd be more willing to believe that the HDPE body had changed shape a little and made for a tighter fit. One of the real advantages that the old Royalite ABS Attex bodies had was that they held their shape so well. I've swapped ABS / HDPE covers around and never noticed much difference in the way they fit.
Are you trimming the cover or looking for another one?
Don
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far