Correct. Only those two bikes along with the Big Wheel, which uses the xt250 motor, works.
Get the internals, the seal for the cover, the actual arm. But do not forget there is a little boss to tap into the engine case for the spring to locate on. You most likely cannot remove it from the donor bike nor will you find it usually sold with the internals. You can buy this part new from Yamaha or make one on a small lathe.