the only diffrence between the grp 21 and the grp based ninja 21 is the grp has a silicone filled crank ninja doesnt ninja has a little more agressive porting with more of a fang cut on the ports as the grp doesnt some of the early grps had the same porting as the ninja i think the early ones were left over ninja p/s they used up gor grp other wise every thing will fit each other dont know about the boss line up hope this helps same thing with the 28 engines also