I had a Holder 12 a long time ago, same year. They're great little boats.
For the mast, you can give it a quick sanding to knock off all the exposed fibers (probably want to wear hand protection to keep from getting splinters). Then spray it with a couple coats of your favorite hardware store spray paint to seal the fibers. You could coat it with epoxy, but it will be much more time consuming and expensive and really isn't necessary.
For the tiller extension, I would give it a quick sand and then wrap it with grip tape (hockey tape). The other option would be to simply replace it. It is the same tiller extrusion that's stock on all the Hobie catamarans. Find the one that's the closest in lenght (Hobie 14?) and then cut it to fit.
For the boom, I think your best bet would be to find a used Laser boom and cut it to length. Laser parts are much more readily available than Holder parts. Not sure if the gooseneck fittings are interchangeable, so you might have to swap out the goosenecks too.