Tire drive is extremely simple though does have some problems.
Otherwise, first up your front and rear sprockets will need to be determined. HP you give at 4-5hp.... so its a big saw?
Most scooters have around 3 HP which allows 65km/hr on a 80kg machine with tons of drive gear drag.... so a 5hp bicycle will be a rocket. Your cycle chain would be too small and snap unless you grannyied it.
Try a 29cc off a weedwacker. Cheap as chips, even new and super light. Buy with a warranty. I have a Honda 4 stroke weed wacker- perfect!.
A MTB disk brake cycle rear disk can be removed from rear hub, modified to install a very large/but light belt pulley with a tiny belt pulley(alloy) on the wacker motor shaft. Or you could cable tie a big pulley to the spokes???Attach to the rim???
Ideally mounting the drive train in such a way that the bike could be used just pedaling with motor disengaged. Another ideal feature would be being able to bump start, rather than get off and pull start.
Pulse and glide should be designed in..... especially considering its a cycle as pushing and pedaling can move the vehicle as well.
Hypermiled with even an inefficient 2 stroke wacker motor, FE with PulseGlide could be as high as 500mpg....maybe 2000mpg depending on how much you want to pedal.
One way to engage and disengage the motor could be like a handbrake type lever mounted on the cycle top tube operating a cable, actuating a cam that moves the motor away from the rear axle thus tightening the rear belt and engaging the motor.
Early mopeds drove the tyre with a nolathane/rubber/urethane bush on the engine shaft. These bushes typically lasted around 1000km and required special tyres that wore too I believe. However, tire drive has potential, especially if a tyre was made with a ring gear incorporated into the sidewall. Personally I like the idea of disk brake hubs on MTB's to secure large pulleys with the handbrake type engagement. The early Velocycles's were difficult to pedal and heavy.
This area is ripe for redesign and innovation, as the plethora of electric hub motor bicycles would suggest. Rocketing down the road at 45mph on a cycle with ordinary frame strength, braking and handling may require life insurance.
What ever happened to Tidalfoce?
IO Tidal Force Electric Bikes by Wavecrest
Rim drive....axle less wheels.....aerofront cycles.....sails.....wings....liftoff....big leg muscles