I do not know Ed. I assume that Horizon sells MVVS gas engines that they make for them, and that they incorporate the latest design changes.
The engine can be recognized by a thicker 9mm crankpin that is longer as well, thus increasing bearing capacity by 43%. To accomodate the larger bearing, the casting had to be changed, as well as the rear cover. If the GX fits this bill, yes, they are the latest design. One of these just won both the Swedish IMAC sportsman and freestyle class 2008.