The OS .18 is a direct drop in for the rusty kinda, I have the old pro .15 model but I think they are the same style, you have to grind the tranny brace because the engine comes with spacers that go in between the mount on the engine and the mount that screws to the engine, so the engine can sit higher so it doesnt rest on the chasis, you need to grind the chasis wider were the pull start and back plate sets, and the motor mounts that screw to the engine you have to grind a good section out were the motor mount goes by the back plate and pull start because the pull start on the OS is bigger than a smaller engines, also your gearing selection is limited because the engine is so big that there is about a few millimeters back and forth for adjustment for the mesh on your spur gear and clutch bell. For gearing the 20 tooth CB I think works with 70-72 spur, the 22 CB works with the 66 spur and probably a 68 tooth never tried but should fit, the 23 tooth CB will work with the 66 spur and maybe 64 spur never tried though, you need aluminum clutch shoes (alum. shoes for t-maxx fit) with that engine or your stock shoes will melt most likely, if you are running stock tires not mashers a 23 CB (robinson racing) and a 66 spur (spur for t-maxx) works nice good accel. and fast top speed, with masher tires the 22 CB 66 spur is good little high geared but it pulles it fine in dirt. After all that is done the engine kicks *****. One more thing I run is the robinson racing lightened vented flywheel for the t-maxx but works on rusty and that will make your engine rev faster. more power