difference in engines?

keep in mind, bikes are lightweight and not really designed for consistant speeds over 25 MPH.. 35 MPH is really pushing your luck..

anyway. a 2 stroke will have more HP per CC, it's just there nature, but they have less torque and they have a smaller powerband than a 4 stroke.... a 2 stroke would greatly benefit from a SBP shift kit to keep it in the power band, but you would need a multi speed bike and an alloy hub

Two strokes can have as much or more torque than four strokes of the same cc's...
Two strokes are lighter and have less moving parts.
Four strokes don't require mixing gas and oil, but do require oil changes.
Four strokes are more tolerant of bad (lean) fuel air ratios.

2 stroke, with a SBP shift kit if you want speed, but you need at least 3 speeds to make it worth while.. actully a 3spd alloy hub with a coaster brake would be perfect IMO.

2 strokes don't have the torque needed to get out of the hole. they have powerbands in the upper RPM ranges and max torque will be in the powerband. hp will be in the same powerband.. so you need somthing that will get you in the powerband quick and keep it there. probably a 3,000 RPM powerband. probably starting around 5,000 RPM