You're going to need some kind of clutch. A centrifugal would be the way to go. You don't want just a direct drive. The engine would always be engaged to the rear wheel, no matter if it was running or not. You'll need to find a clutch that will either fit or can be adapted to the motor shaft. Depending on many factors such as tire size, engine rpm and torque, a gear reduction of some ratio will also be necessary.
You might want to explore a friction drive as opposed to chain drive. That will simplify the project but the above suggestions will still apply.
Check through some of the information found in our DIY section for non-kit motor installations.