The OS15 should fit without any problems. Make sure it has the "standard crank" and not the "SG" or "short" crank. If you are going to change or upgrade the engine, get the HPI aluminum flywheel and the HPI racing clutch. That combination is almost "bullet-proof". Is is much more reliable than the "stock" clutch. If you are going to upgrade the engine, I would recommend the new OS CV-R .15 pullstart. Upgrade to a 15 tooth clutch bell ( you will need the adjustable aluminum motor mounts, less than $4.00 ) and you will be able to do "wheelies on demand". The OS CV-R is a very powerful, easy starting and reliable engine.