I believe your problem is not voltage, but the wattage rating of the motor. Most of the specs I could find indicate your speed is about what you can expect from a 250W motor, particularly if you are a big guy and the bike is heavy. I think your best solution would be a 500W 36V motor and a controller adequate to handle that package. 250W is not adequate for serious riding or hilly terrain.

I run a 36V 500W geared motor, 26A controller, 36V/12AH LIFEP04 battery on a 700c hybrid bike. I weight about 220. The system is limited to 20mph (also the Minnesota limit) which it does with ease, and feels like there would be more if not for the limiting. Low PAS mode on level is about 14mph. High PAS mode will max it out. Hills are no problem, and depending on the terrain I have about a 30 - 35 mile range.

I once met a guy on the bike trail with a 2000W motor and massive 48V battery pack on a heavy 24" trail bike. He claimed 40+mph on the flat.