Hello Piero
just for something different, a 13 tooth option may replace the Norton standard ist gear 14 tooth pinion. it all depends on what access u have to engineering gear and how many mates u have that may help. it has to be made but is not that difficult, the biggest hurdle is access to workshop and heat treatment gear.
the 13T pinion fits straight in with no modifications, can be used with a standard second hand 28 T gear, but a new 28T gear is recommended,
the 13 tooth pinion is far superior regarding strength and has far less tendency to pit and score. the standard 14T pinion has not been modified, and shows severe undercutting. I reckon the ratio change would give a touch more off the line for those who like harsh acceleration.
if you like PM me for more information.
Bradley