Don't get rid of what you currently have. You might regret it one day! I suggest you pick up a C50 engine and swap over the parts. I'm pretty sure they are compatible, but worst case would be that you use either the whole C50 engine or the C50 bottom end and fit your ST top end to it.
I did this the other way around many years ago and converted a standard ST50 to a 70cc by using the bottom end of an SS50 engine mated to a C70 top end. This gave me a 4 speed 70cc bike which, for its time was pretty nippy.
Just out of interest, why do you want to change it back?
