There are really three major parts to this, all of which were found in my home. The most important part is the electric motor which can be salvaged from any number of electronics. This particular one came from a non-working disc drive and the motor was used to open and close the tray. I left the gears attached and just cut the plastic around it so everything stayed intact. Also needed is an empty jar of peanut butter with the lid, and a 100 recordable DVD casing, which needs to be the type where the spindle unscrews from the base as opposed to the type where it is fixed in place. You'll need a few other odds and ends such as a little plastic ring from the stack of recordable DVDs, some cocktail straws, four pieces of short (3-4 inches or so) galvanized wire, and something to secure the motor down later in the process.
Nice what you did with gears, lower gear ratio should provide enough torque to stir the ice cream when it is almost hard.
Here is a nifty battery guide that I just found online : http://www.freewebs.com/dacrochet/Battery_guide.doc
Reference: http://www.webexhibits.org/butter/kitchen.html