Install charging on go kart engine?

I have a motorized cooler that came wiht a 1HP 30cc engine. I replaced the engine with a 2.5HP 89cc engine I bought from Harbor Freight. I installed a CD deck and 4 speakers on the cooler with 20 NiMh D cell batteries powering it two sets of 10 in a series to get 12V. Anyways I would like to mount a generator or alternator on the motor but don't want an automotive one since its so large. I would greatly appreciate any help on how to go about doing this. Would I be able to use a starter from a Seadoo as a generator? and if the batteries are connected to the starter wouldnt it just spin? Thanks in advance for your help!