38 tooth sprocket?

Primary Reduction is 3.227

Engine RPM varies... I would guess a blaster with stock porting and a pipe revs to maybe 8K ? not very high, a ported engine with all the right goodies should rev to 9000 or so.... Whoever ports your jug should be able to tell you what rpm its capable of pulling. A built stock bore ( think national blasters ) can rev very high, just comes at a price or rideability and reliability.