HP?
Anything bigger than an 8hp will be overkill, and that is with a two stroke (assuming you can find one for sale). With a four stroke, a 6 will be more than enough. If you are buying a new engine, you will have to buy a four stroke, as that is all that seems to be for sale these days in the smaller engine sizes. The four stroke is superior in many ways to the two stroke, but it does have two big drawbacks for you. One: it will weigh significantly more than a two stroke. Raising the outboard bracket is hard enough with the two stroke; its even more difficult with the four. Second drawback is that you will probably have to replace the outboard mounting bracket. The OEM brackets are not acceptable for four strokes; the weight of the four stroke might be low enough to be within the bracket's specs, but a four stroke generates more torque than a two, leading to more stress on the bracket. You do not want your bracket bending or breaking while motoring. The big issue with replacing the bracket is finding a four stroke-capable replacement with the same footprint as your OEM bracket. If you can't, you will have to drill new holes in your transom, and maybe even build a new bracket spacer block (that wedge-shaped hunk of fiberglass on the transom that puts the bracket on the proper angle) to support the new bracket. What a pain. When I repowered a few years ago, I wanted to buy a four stroke, but all of the above lead me to buy a two stroke (apparently one of the last ever made!).Whatever engine you buy, you must buy a long-shaft model. If you are expecting to go through any sort of rough conditions, buy an extra-long shaft model if it is available.