From what I know about MOSFETs, from experimenting with them on my PSU array, the driver doesn't necessarily have to be beefed up.
The heat mostly comes from having a large current pass through the MOSFET, which has a certain resistance, even when on.
If you think about it... 25 amps through a 1 ohm resistance creates a 25 volt drop. Obviously the MOSFETs have a lower resistance than that. The one I found has a resistance of 0.004 ohms which, at 40 amps, creates a voltage drop of 0.16 volts. It's that small voltage drop that heats up the MOSFET: P=V*I so the power dissipated by the MOSFET is 6.4 Watts!
Otherwise, you should be aware of the driver characteristic: what driving voltage does it supply, and how fast is it? Then you have to match that with the MOSFETs response time.
So your first step ought to be to identify the existing MOSFET.
I can help you search through Linear Technology's line of MOSFETs, for an equivalent.