Gen II engines have a cast iron cylinder liner vs. the chrome plating found on most of the other kits. The Gen II has a flange mount carburetor instead of the clamp mount carburetor to eliminate a source of leaks. (the joint between the carb and the inlet tube)
Of all the HT engines I have seen. the Gen II's are the best. Very few were imported here before some EPA paperwork screw up prevented the importation of more.