Kewl. ScottyB at Carts Unlimited, a Site Sponsor, sold me everything in my sig.
He or others can recommend a good controller, as mine is "dated". A new controller can run at 36 volts now, and 48 if you decide to upgrade later. Your motor can take this just fine. Your motor's Hp is about half of mine, but I'm pretty sure it's a good bit more than stock AND bigger tires will need more amps.
With a series drive, volts = speed and amps = torque. With your bigger tires you traded torque for speed, AND it's pulling more amps than your stock controller can handle.
Can you tell us the make and model of your solenoid and where you bought your FNR?
Here's a link to ScottyB's website. It's a good place to read up on what each component does, and will help you figure out what you have vs. what you need to get it to do what you want.
http://www.cartsunlimited.net/
How new are the batteries? If pretty new - a few months - one option could be to put one battery under the passenger side and another in the bag well, to go to 48 volts now.
Amps = heat, both for the motor and controller, so if you increase the voltage, you decrease the amperage, for the same amount of work (+\-).