I think Maxtnet must have had the OS FP series in mind, the bushing 40 backdoor Fox's easily outpowered them. It would be a close second to an OS FSR. But Fox had ball bearing version's would easily outrun the FSR. In fact the Fox Quickee 500 would easily out run any OS V series. Fox engines have a few faults, but being a weak runner is not one of them.

The FSR series came out about 1975, the backdoor Fox's began either the late 70's or early 80's. But not sure if the butterfly carb Fox was one of the first backdoor version's, or a bit later.