Well, with some engines it is very possible to do some minor upgrades, but if you do not have the engine yet I would just find what you are looking for off the bat. I have had a Johnson 9.9 and all it needed to become a 15hp was a carb swap, and it had quite a bit more power. But, what it really did was shift the power curve from lower end to upper end. Didn't really seem to have much more lower end torque, just more top end. Planing speed was about the same. That's the funny thing about getting more horse power from the same displacement block. You will notice more upper end speed than planing torque.
Give us a serial number of the particular motor so we can see what the upgrade entails.
What size boat are you getting a backup engine for? I had a 1970 4hp johnson for a backup on my 16' aluminum boat with 65hp mercury. It would have gotten me home on a windy day at the lake, but if I were stuck downriver Id have been lucky to make it back up river with that tiny engine. Now, when I put my 9.9 on the same boat, it actually PLANED IT OUT!!!! I could do 8-9MPH with that little engine fully planed. Blew my mind!! I will try and post the videos for you to see what I mean.
