Hi,
It sems your motor is draining a lot of current, the mosfets don't need to heat up in order to get fried. One way to check up is to connect your motor directly to a power supply thoraugh an ammeter, and check your current, at starting, no load and full load...
Normal mosfets would withstand currents upto 4 or 5 amperes max, special mosfets would be needed for higher currents. Starting current is the most dangerous, so you'll have to multiply your full load current by a factor of atleast 3 to get your mosfet rating.
Check the TIP 147 mosfets, they can handle upto 10 amperes continuous load. Make sure you equip your circuit with a reversed diode in order to handle reverse currents.