jet200 carb & exhaust set up

[joda]

my jet200 has a sterling pipe fitted at the moment, im not to keen on this pipe as it always seems you need to rev the tits off it to get anything out of it!

i have been given a clubman with vw tailcan and plan to fit this

the carb is a standard 22mm jetex as far as im aware so what jetting should i go for?

also, how does one measure the exhaust size, eg: 43mm 44mm etc and what effect does this have on the performance?

excuse my ignorance its mostly vespa exsperience i have