If you are asking if there is a 48-50cc 4 stroke similar to the gx50, there no alternative yet as far as I know. I'm sure huasheng will copy it, and maybe make it a bigger bore like they did to the gxh50 with the 144f.
Edit: looks like I'm wrong, huasheng has already made one lol. Not sure how easy it will be to find one for sale.
Double edit: oops thats a 2 stroke lol. Just give it time, hs will make a gx50 clone.