6 FETs dissipating 56W each?
A cheap garden variety 60volt 50amp TO220 FET will have Rdson of about 20 milliohms, and at a sensible 20A operating current will dissipate I2R = 20*20*0.020 = 8 watts per FET!
6 of those FETs will allow 120A load current at maybe 48v PSU voltage, so that's close to 6kW motor power. And 6kW is PLENTY to run your electric go cart unless of course you decided to enter it in the land speed records.
How did you work out 56W per FET?