I also started with a mistral maui, about 3 years ago, it's a great board for learning. it is possible to fit a modern rig to the board, I did a modification to the sliding base to take a standard mast base. If you post some pictures it would be easier to see what you are missing. Are you in Sydney? I might have some stuff you could use. Cheers

The main problem with any older stuff is to make sure you dont loose any bits. The mast foot for these boards are pretty rare and really hard to find. Some battens in your sail would help. The new boards are much easier to learn on and progress a lot quicker but keep at it and you'll soon be riding waves, well done. It alos looks like you have the original mast extension, the mast side of the sail should also be nice and tight, you can wind out these extensions like a screw and make the mast longer, look at some clips on you tube about rigging. Your sail wont go loose on the leech but should give you some idea of how to rig it better.

I know and I think wind and water was just about the same in the eighties. And this is not the first place where I hear that it is good for a beginner. I also don't think I should pay a lot for a sport I barely know but want to try.

Consider getting a clamp on boom. These booms have been around for over 20 years so you should be able to pick up a second hand one. A clamp on boom will firmly attach to the mast and won't wobble around. The sail you have does not appear to need a very long boom so you should be able to pick one up from a local windsurfer for very little money. Downhaul the sail a lot more. If you can't downhaul the sail because the mast is too short you will have to look at get a mast extension. Using a new style extension and universal will probably be an issue with your mast track. They won't be compatible. So follow powersloshin's advice. Enjoy the windsurfing!