Everything Denny said is right on. In summary if you go with a bracket on a 20 cc make sure you:
1. Get as light weight a motor you can.
2. Get a bracket with as large a flotation chamber as you can
3. Keep the weight as centered in the boat as you can (fuel tank, batteries etc...)
4. Go with the minimum set back that will allow the motor to tilt up.
5, Get trim tabs (however I hardly ever have use mine)
If you go about it right then you wont be disappointed in the way the boat handles. Just ask Bigshrimpin how nice the boat in the pic below handles. And backing up is not all that bad.
LIFE IS GOOD
strick