I've talked to Ricky Shi at Boy Go Fast on eBay several times and our problem was settled with no troubles and I have since bought from him several times, rings, gaskets and the like. I even got a CNS V1 carb from him that works great. Beware of any kit boasting the CNS V2 carb. It has a red cover and works like crap. Most polite thing you can say about it
In all actuality a 49 might be best. For 1 thing they run smoother than a 66. To make the 66 all the Chinese did was bore the jug and make a bigger, heavier, piston and not balance the new setup. Second thing is that in most states it's as large as it's "supposed" to be with 50 cc being the legal limit although you can't tell the difference externally.
With the criteria of hillclimbing ability one of your first mods should be a bigger gear to make the climbing easier on the motor(and you) On my 28" wheel I use a 56 tooth gear, on a 26" I'd say a 48 or 50 with the preference on the 50.
At the same time I'd suggest a Sick Bike Parts expansion chamber with the add-on muffler. Best single performance part you can get.
With a 49cc motor, bigger gear, and the SBP pipe you'll be conquering the Tennessee hills in no time