[curtis73]

So its basically a hot rod that is limited on speed so younger drivers can operate them legally? Sweet idea.

As far as the rear suspension, you could leave the existing suspension in place, but bolt or weld in a solid piece of metal where the shock used to be. That way it would use the original suspension to properly locate the axle front to rear and at the right angle, and you're just using the steel to prevent up and down motion. The stock shock mounts might need to be strengthened a little since they are only designed to take the load of the damping of the shock; not the whole shock of a bump.

For speed control, you could gear it so that maximum RPM is met at 40kph. That is not the best way since any time you're at maximum speed you're also spinning the engine really fast.

Interestingly enough, the tractor name made me think of another idea. Have you thought about using a governor for RPM control? Agricultural Tractors don't use an accelerator system, they use governors. You set the RPMs with the lever or foot pedal, and a centrifugal system senses load and adjusts fuel delivery to keep the same RPM regardless of whether or not you drop a plow in the dirt, go up a hill, or have the clutch in or out. You might be able to use an engine that comes in both applications and use a governor designed for it. That way you can set it for a certain max RPM and tune your gearing accordingly.