One rpm??
I believe most small electric motors turn at 1725 so you are looking for some serious gear reduction!!
I would start looking for an old tiller with a vertical shaft engine. This would have a worm drive. Don't off hand know how fast the tines turn but the engine would be 3500 or so and since you would have a motor at 1725,this would cut the rpms by half (as opposed to what they would turn with the gas engine) for starters. Any further reduction could be obtained with chain and sprockets.