I'd intended to get a 66, but I accidentally bid on the wrong eBay auction and won. I'm also lucky I haven't had any problems with this kit, because I'm sure I wouldn't have been able to get any warranty service from "random eBay seller."
So I was a tad disappointed in my lot, but it turns out that the 50cc goes just fine. Plenty fast, powerful enough (and I'm not a small guy). I rode with a friend who had a 66, and he couldn't keep up with me. If you get a 50 (49, 48.5, whatever), you won't be disappointed by not having a 66. I've also heard that the stock carbs are really made to work best with the 50s, and that the 50s rev higher than the 66s. Higher RPM means higher top speed.
All that said, if you're really interested in a 66, get one. No cop is going to know the difference - or even know that there's a difference to be had - and it's not likely any of them will care. Just use some Goo Gone to remove any identifying labels.