some maths
internal redux is approx 4:1
10 t sprocket and 44 t chainwheel
gives about 18:1. thats much the same as a stock ht.
at 60kmh, a 26" wheel is doing roughly 475rpm
475*18 = about 8500rpm.
i believe a morini will spin at 11000 rpm.
calculate what ratio you will want for say, 70km/h @11grand... it will be around the 20:1, maybe more... doesnt have to be terribly accurate, just an estimate...
so using either a 24" wheel, or a 52 tooth chainwheel...
hopefully that would bring the low speed side of things(<30km/h) within the useable rev range...
once again, it does depend on the exhaust.
you can have a wide band pipe that produces moderate power,
or a very peaky pipe that produces a LOT of power...at full tilt. nothing down low. which isnt terribly useful without a gear box.
id basically start with a 44 tooth chainwheel and experiment with them, as they are fairly simple to change...depending on the mounting methods. bigger will reduce top speed but give more pulling power, and smaller will do the opposite...
the problem with a shift kit is finding one that will cope with 6 or more horsepower! reliably!
and personally, at the price they are... just not worth it. the engineering of mounts etc needs to be spot on or youll die...