James external reservoirs ( piggyback and hose and reservior type ) are not permitted in pre 75, as far as I know. Piggybacks from 75 > , with or without adjusters , no problem. I hope that answers your question.
To answer your question , what is a scooter shock ? Well its the same as a motorcycle shock , exept that they are usually short , with sometimes different mountings like studs ( Peugot) Mc Pherson mounts ( Piaggio) or diiferent size clevises instead of eyes. But the components of those shocks use the same modular systems as we use for BMW or any other bike . So all the strenght testing on parts is done for big bikes and therefore well truly suited for scooters , or in your case VMX.