doopsx3

yep MAJOR engine mount changes required to put a CRF50 based engine in a ST90....the ST90 is a different base...largest OE displacement I seen for those is 110cc...I had originally was going to use a ATC 125M engine in my ST but the engine did not align up and it was heavy...I went with a Lifan 138cc...you could go with a big YX ...maybe one of Firepowers 192cc motors but you need to be raedy for some fabricating and bracing...the pressed steel frame of the ST is not gonna take the stress for too long....what ya can do tho is get a hi-compression piston from Firepower and a hi-lift cam from DRATV..maybe even slide a 110cc jug on it...send your head to Firepower and let Terry play with it and maybe open it u abit...manufacture a free flow exhaust....but ya may have transmission probs like 'yella said....look in my profile and check out some of my pics in my ST90 'Tard album