20 HP, 40 hp peak seems to be a common number. Don't worry much about torque. Any electric motor you find will have fistfulls more torque than the three pot engine that you pulled out. Besides, you have 5 gear ratios to choose from. Before you go out buying ****, think about the future. 72 volts is a very low system voltage... Solectria's are 144 volts, Tesla is like 320 volts. I worked on a gel lead-acid/WarP car that ran at 177 volts...
You may want to upgrade at some point!
The lowest voltage component in your system is what will hold you back. You are building a 72 volt car, but you may want to spend a wee bit more for a 96 volt charger (Delta Q makes a nice one), 96 volt contractors, 96 volt shunt, 96 volt fuses, etc...
Going overkill on the component rated voltage will make voltage upgrades easier. Say you are not satisfied with the car's performance, if your components are appropriately sized, you just pop another battery in and away you go! Also, using a higher system voltage will yield the same performance from smaller, lighter wiring, while drawing fewer amps and getting more range.
Just something for you to chew on
~Jimbo
P.S. Could the mods move this to the FFF forum?