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sailorman,
Both the Yamaha F60 or F70 will make an old Sport 15 scream. The main difference between the two motors will be the larger one will out hole shot the smaller one, and the smaller one will be a little slower on the top end than the larger one. If you are going to be carrying a load where the hull is at full capacity at all times, you might want to opt for the F70. It will handle the load easier.
The old Johnson might not be the best thing to continue to throw money out. I have the same boat, and after doing many things to my old Johnson 70, I finally sold it. I still have not repowered our Sport 15 yet, but when I do, it will be the Yamaha F70.