You will find these in about everything.. I would suggest looking for the 12 hp K-series. You are about the run the 18 parts within the block and make a considerable amount of power.
You will find these mostly on John Deere, Wheelhorse, Cub Cadet, Case, White, Bolens, Sear.... the list goes on..
If it is a garden tractor before the 1990's there is a good chance it will have a Kohler k-series engine on it..