I don't know if this will be of help or not, but i believe i saw one of these for sale at a local bike shop.
You might call them to see if they have or know of any parts for sale.
Desert Sports Co. # 1-800-547-9888
Another place i know of that has worked on these old Harleys is:
Poor Richards Cycles: 1-505-324-9423
I just sold a 78 sx125 this fall. While I was doing it I came across MotoItalia they are in California and had a complete catalogue of parts for the 125. All origonal in the origonal boxes and at decent prices. Wish I could help you more but I gave all the info to the guy that bought it. Try an internet search I think they have a site