Go to 15mm - you really don't wanna go over that unless you have a 70cc Head on there, cause getting the mix right is insane-hard with a carb so big mated to a head of only 50cc.
Now if you do have the kit, go 17mm-18mm - but for the 50 ? stick with 15mm.
Make sure to get a couple of jets for it too, you might need em.
-R