I weigh 14 stone too.
But my xk magnum bike with 2 speed auto Za50 engine on it weighs a lot less then your shifter bike.
I got cheep estroil pipe and tm24 carb and 8 petal reed cage, k&n filter and my bike goes 55-60 mph and has for years
So 50-55 mph is a good speed for reliability with smaller carb and cheep pipe
Why make your bike go super fast so it just blows up ?