i think tinkering with the gearing would be a bit difficult, im assuming the wheelchairs are just a work driving a planetary gear, right? well if you put a smaller gear in then you will have to move the motor around and fiddle with it to get it to mesh properly.

i think the original mower was simple enough... your not exactly going to be out there all day playing with your mower are you? (well if you will then you need to get out more :P ) so a couple of 12V batts would be adequate i should think, or use the wheelchairs stock batteries as most wheelchairs can usually run around for a couple of hours before needing a rehcharge.

even better if you have a ride-on mower! [>:] a couple of mega servos and your in buisness...