[Al Skierkiewicz]

I would like to add to Richard's excellent discussion that the brushes in the CIM motors are not positioned or rated for generator use. The diminishing returns on this project are weighed against the Chalupa as a generator. Remember also that there is heat generated within the motor during this process. The efficiency losses are turned to heat and this motor is designed for intermittant duty (Read: No provisions for cooling).
Mike,
An alternator is an often used device in this type of application. To use an alternator, you need a variable current supply to power the field coil. The output voltage/current is directly affected by input speed and field current. Many people just remove the alternator and voltage regulator from a junk car. They use a battery to both supply current to the field through the regulator and to smooth the pulsating DC voltage from the alternator.