Hi worksmanFL,
You will need a drive system, exhaust system, motor mount, chain, throttle control [with kill button], throttle cable, gas tank, fuel line, fuel filter, rear wheel sprocket, and misc. bolts, nuts, washers, etc. Sadly the support equipment will cost far more than the motor, and will be hard to justify. I know of many that ended up spending far more than actually buying a complete 4-stroke kit, because parts are much cheaper when bought in large quantities.
The drive system can be fabricated in many different ways. You can use a jackshaft plate, bearings, a clutch on the motor crankshaft, a primary chain, sprockets, secondary chain etc. You can also purchase the Q-Matic drive already setup for the HF motor [often cheaper than buying all the parts and fabricating a "homemade" system]. Considering the clutch is wide, understand if located on the motor crankshaft the motor will be very wide, and will require some serious attention to the pedal arms clearing the motor.
If you use the Q-Matic drive, then the current available 4-stroke motor mount can be drilled to fit the motor, whereas other drive systems may require a custom mounting plate to aid in "off-setting" the motor to line up the rear chain path.
The exhaust port is in a poor location, and most route the pipe up and towards the rear, I personally don't care to have the hot pipe near the seat, therefore I suggest the pipe be routed to the front and then down under the motor.
Another issue to consider is the final over all drive ratios, since the motor is a low RPM motor, special attention must be paid to "get it right". You need a final ratio of 12 X 1 [11.5 X 1 to 12.5 X 1] to take advantage of the motor's torque, and overcome the narrow RPM range, if lower or higher it will require pedal assist, and or a very low top end speed. A few tried 9 X 1 and had to offer serious "man power" to get in motion, and found the motor struggling to climb small hills. Some tried 18 X 1 and found the motor wound out and the top speed under 20 MPH.
Hope this information is helpful, as we have a lot of experience with the HF motor, and several of our authorized dealers have been using the HF motor for several years, in fact I think Jim and Chris Davis in PA were some of the first to use the HF motor on a bike.
Currently we are using the HF motor to test our prototype 2-speed drive, and is a great candidate because of the narrow RPM range.
Have fun,