Before jumping the gun, find out what is wrong. As far as repowering, You will have to stick to Merc or ( I can't remember who) a Co with the same bolt pattern on the transom. Some other Co. have a different bolt pattern which would entail plugging/drill new holes. Like mentioned, you might need all new controls (throttle, gauges etc) which are typically $500 and up. New/rebuilt power head should be your cheapest option. Going to a 4 stroke, your going to be adding weight to the transom. Depending on the year of the boat, the 4 stroke might be to heavy. The boat will not respond the same. You typically would need a new/different prop going to a 4 stroke also. Due to the performance/weight of the 4 stroke.