For most people the two-stroke engine was a phenomenon of the fifties and sixties. It is often regarded as being noisy and smoky. The powerhouse of Grand Prix racers, small motorbikes, mopeds and outboard engines for boats, some feel its life is almost at an end. Gordon Blair has devoted his working life to developing the device and feels it has yet to reach its potential. Much evidence exists to support that opinion. Reporter Mike McKimm. Producer Davy Sims (R)