Pretty sure it's a 19 Hubie (it's what I took off mine anyway). I got a 20 from either SRM or Draganfly, I forget which. There are larger sprocket options if you've got the power and not the weight - but some of them require fitting rear sets as there isn't the room for a very large sprocket with the standard footrests in place. I think 21 teeth is the practical limit, someone else will know for sure. My engine is in standard Flash state of tune and I live in the middle of nowhere, so town riding is rare and the extra gearing is OK. For me, it makes cruising on empty roads better and gives just a little extra length to 2nd gear, which I always moan is too low in STD boxes. But I think that's just me.
On mine, I found the extra tooth gave me a chain-pain - had to fit a cranked link in the rear as the primary was also new and the gearbox was fully forward. The brand-new rear was just that tiny bit too tight to get the correct amount of slack. After a fair few miles and a small amount of adjustment on the primary, my newest new rear chain is the correct 98 links and fits nicely.
