Whoops, Mustang Trail Machine engines had 5.75 hp Briggs engines. And after thinking about the wheel color and shape, they are not Mustang wheels, but Hawk wheels, which were used on Hawk's Pony Cycle, Simplex Seniors, and a number of trail scooters including the Rose Rambler, etc. Hawk supplied 12" wheels, Continental engines with lighting coils, and their Deepco Maximatic torque converter to other manufacturers. Almost too nice to be homemade.