1975 ST90 with the weak stock engine.
A 4-speed manual clutch with the provisions for an oil cooler was purchased from ebay smilar to this 125cc Lifan
The ST90 engine mounts to the frame with 4 bolts, instead of the usual 3 bolts that the other Honda horizontal engines mount with (CT70,XR50).
The lower rear bolt holes can be used with a spacer, I made brackets that bolt to the engine with a hole to match the frames stock mounting hole.
Exhaust is a custom bent 1-1/4″ Stainless steel header matched to a ZZZ muffler.
Position of the oil cooler is a bit of a problem. The best and cleanest location is to mount it directly in front of the head using stand offs. We welded 7mm X 1.0 thread nuts to one side ofa 3/8″ tube, and an M6 nut to the other, and simply bolted the cooler to it.
more to come