Welcome to miniriders, plenty of info on here on what you could do. As for where to buy parts have a look down the left-hand side of the page under the heading "Cool sites" that should give you a good start
50 cal stuff = junk stay away
carbie is too big
shock prob wont fit
Yoshi pipe will fit , but maybe better suited for big bore kit.
What ya wont to do to it ?? basic stocker build ?(adult proof) or you wont full blown 50?
And if ya wont good parts prob going to have to shop overseas as is very little parts here in aus.
And mad to build a full blown 50 from scratch these days as could buy a 2nd hand done ready to go for half the price or less.