As noted, first you're going to need a motor mount. For my 2 cents, you should have it mounted by a dealer to make sure it is securely fastened to the hull. There is a bit of torque applied to the hull when the engine "goosed" and you don't want the motor mount coming loose. I've got the Honda 2HP air cooled engine that will push my 170 close to 6 MPH at full or nearly full throttle. These engines are pricy (around $1200 for a new one).
You might be able to find a decent used engine if price is a consideration. However, I highly recommend getting one with an idle feature. Slow the engine down to idle and the prop stops spinning. Since these small engines typically don't have a shift lever (forward, neutral and reverse), IMHO it's very important to have the "idle" feature. Otherwise, as soon as the engine is running, you're MOVING and you might not be ready for that.
I tried an older 2HP engine without the idle feature and I found myself bumping into other boats when I wasn't ready to move. This kind of engine works fine when you're OUT there, but starting from a dock and other congested areas, it's not the best thing to happen.
I use my engine to get off my mooring and when I get about maybe 300' out, I pop up my main and only then, shut off the engine. I then tilt the engine up to that I'm only using my ship's rudder.