technically your bike is not restricted, yamaha built these bikes to do 30(ish)mph (i had an rd50mx). you can however make it go faster by adding a few after market parts like:
an aftermarket expansion pipe, (like a "fresco" or "the big one") to improve gas escape.
a s+b or k+n filter, to improve breathing.
ledar kit (if you can find one) works in conjunction with the above two items.
hytech or similar reeds and open up the reed stops as far as the block will allow.
all this may or may not add 5-8 mph depending upon set up and sprocket sizes.
the best thing to do is get a big bore kit 60cc or 80cc this with the above mods can add 10-15 mph.
tuning is an option but for the cost on such a small bike is it worth it (unless you intend to keep it)(i have just bought a jap import 1993 jap import tzr50r 7.2 ps should be good for 60mph according to jap web sites its a project)
hope this helps you out just type dt50mx in google and surf around for the stuff ok