Almost any rings can be purchased from Hastings Piston Ring's site.
Many older Mazda diesels were Perkins design with pintle injectors. One example is the 2.2 liter diesel that came in the USA Ford Ranger. I have no idea if those injectors fit any Hercules though. Mazda used that Perkins 4-135, 4-154, and 4-182 (same as used in many cars and tractors). 1960s Jeep and Checker also had them.