Never attempted a on/rd Engine in Buggy/Truggy. The Gear ratios would be different in a off/rd VS on/rd. I know the timing is slightly different between a jp-b5n mod and a jp-r03 mod. The 35 PLUS 21 IS A Fuel hog!Most .21 on/rd Engines that i have seen don't go far on 120cc. The .21 on/rd engines don't get as good of fuel economy as the .21 off/rd engines.