Do you want cheap or low cost? Low cost would be any one of the good used engines listed for sale here. A G-38 should fly your plane just fine and they are usually low cost. Also a used Quadra 50 or US engines 40 would be in the same price range and also work well.
If you want cheap then any of the numerous Chinese imports in the $150 to $200 range fill the bill. They are almost throwaway their so cheap. I haven't yet seen one worth wasting my money on. If you want to spend more there are several higher quality Chinese motors available but I don't think they qualify as cheap.

Larry