Cool!

Like I said, old windsurfing gear is BACK! Pics of the entire board, please!
What exactly do you have? A board, sail, mast, boom?
Do you have a UJ (Universal Joint)? Do you have a mast extension? If so, please post photos showing them from the side, top and bottom, and then people will be better able to help.
If you don't have an extension and a UJ, you're gonna need them. If you have to get a *ahem* more "modern" UJ than the one originally designed for that board, somebody with a nice shed will probably be able to knock something together which will allow the UJ to connect to your board.
Btw, how many fins does it have?