I've done exactly this project with a motor from a weed whacker... but I don't remember the horsepower (it was probably less than 5 though)
The basic problem is gas engines like high rpm to provide high torque. In practice once I got going about 25kph, I could get cruising speed to happen, and accelerate a little as well)
My clutch was a brake cable attached where one brake normally would go, my transmission was a rollerblade wheel attached directly to the shaft of the motor. The size of the wheel is critical in the distribution of power to your bike (it acts as a gear, but doesn't always start with a useable ratio, I had to take a little material off of one I used).
I physically pressed the rollerblade wheel using my "clutch" into the rear tire by having the motor slide at one contact point(nearer the shaft) on the motor mount, and fixed (but freely rotateable) at it's other mount point (farthest the shaft). Using this method, I used the rotating rear tire to start the motor (no pull cord for me).
For safety reasons, I included an electric kill switch located at one handlebar.
Eventually I had to retire the contraption. The vibrations shook the bike apart eventually causing faliure of many things (it was a drum brake bike, and the drum was shaken apart). You could prevent this, but the engineering time would be significant.
My top speed was over 75kph downhill (I often passed regular vehicles, and that does get one some rather surprised looks), and this was clearly a dangerous speed (I wore no helmet).
The gas was excessively cheap, and your 18 mile commute would have cost pennies.
I often rode it with the engine off, and the weight was not too much trouble.
Having done all that (and I encourage you to give it a shot, and let us know how it goes), I would highly recommend however that you do an electronic conversion instead.
Electric motors like to give you high torque at low rpm (which a lot of biking demands), and doesn't totally piss out at higher rpm. The vibrations would be less, hence less repair worries, but the final weight will be much higher due to batteries.
My drawing board stage project is for an electric bike with an enclosure so I can drive in the rain, and have better aerodynamics, and it will be a recumbent... If I only get the time. My commute is slightly over a marathon.