I can't remember right off hand the size of that Twinstar carb, but I remember it bolted right on and connecting the throttle cable to the carb was also dead easy to do, I used to sneak the carb off this bike to put on the kart fairly often when I was a kid and never even massed with the jetting or adjustments other than idle speed, mainly beacuse I needed to sneak it back onto the bike before my dad noticed it was on the kart or off the bike...
For that year model, the Twinstar 185 only made about 10hp and used a single carb so I'd guess it was around 20 to maybe 22mm and it definitely made a difference on the Tecumseh engine. Too bad we didn't have easy access to billet rods back then...
As far as carb size selection I'd say keep it around 22mm and you should be ok, you could probably go up to 24mm just fine. If you can find a CV carb between 22 and 26mm, that would work really well since it'll only open up to what the engine can use to prevent bogging and they accelerate very smoothly, even if you snap the throttle to full from idle.