I'm not sure about coming up one that's "bolt-on" ,but, I'm using a walbro carb that I got from pocketbikesunlimited.com and it works great. I did have to drill the manifold provided out to fit the "stock" intake manifolds diameter and drill/tap one hole through the "stock" intake manifold to match a hole in the carborators manifold. It was pretty easy and it works grrreat.

It was called a 16.5MM remote purge carborator, was for any of the 2-stroke 48cc pocket bikes and I just chose the 48cc model, though all of them are for 48cc engines the intake manifolds differ a bit bike to bike.

good luck...