Re: Considering motorizing bike and power questions
50 cc is adequate for the job. On the flat, the resistance comes mostly from wind resistance, not from weight of rider and bike. With a good system you can get 30 mph on the flat.
Hills are another matter. If you have big steep hills to climb, you will need to gear down the motor. That will reduce your top speed on the flat to probably 20 to 25 mph. Pedal assisting on the hills will also help somewhat.
But make sure the bike frame that you have is strong. And make sure you have really heavy duty wheels.