a little background.. four into three wont go.. fourspeed box wlere not manufactured pre 65 and some clubs did not accept them in trials many clubs though have a pre 70 two stroke class for which they are eligible. Yorks classic was certainly amongst the former very much a three speed only.. however a motion was put at the agm some few yaers ago and the two stroke class became a pre 70 two stroke class and thus the 4spd became elgible and approx. a dozen bantams from home brews to 'works draytons can be seen most weekends in Gods country.
the suzuki piston is thought by many to be essential but i cant notice any difference it makes swapping between bikes, electrix world ignition is a valuable mod ( other ignitions are available) and swapping the gears inside the box is also of high value in a 4spd but no good in a three..the best mod is time and patience.. get that carb spot on and they stop and go with barely a whiff.. no figures to guide though as a brilliant carb on one bike might be a crock of ## on a bike of the same spec.
i have a drayton bantam with roadholders and falcons with a std engine and frankly its brilliant goodness only knows what a decent rider could do with it but more than one has ridden it and offered strong money..