Taken from a search on here,,,,
OK. The only "Real" differences in the early and late FA-120 engines is the ABC cylinder and "Normal" cam on the early models. The later ones have AAC cylinders and the high lift cam.
Easy ID for the ABC/AAC cylinders is the cooling fins. Look at the cam box. If the fins reach out over the box, it's ABC. The AAC cylinder has smaller fins, the cam can be removed with the cylinder in place.