Look for a used fractional hp gear motor, you can easily go down to a 1/6 hp. How you are doing your final drive (ratio) will determine the gear reduction you need. Example, if you are laying the barrel in a cradle, and the wheel on the drive axle is six inch dia. and your drum is twenty four inch dia. you have a 4-1 reduction, so a 1725 motor is now down to 431 rpm. So, if your goal is around a 10 rpm drum speed a 40-1 reduction on your gear motor will get you close at 10.78 rpm. Different motor speed and diameters of any parts will change your final, just use the same principle and plug in your numbers.