I threw away one of those uni joints a few months ago!

I seem to recall that the socket is too small in diameter to accept a Mistral or Tyronsea type uni pin. You could perhaps either epoxy a washer into the mast slide to take a Tyronsea "deck plate", or simply buy a block of bronze or stainless with a hole threaded big enough to just fit inside the track when dropped in at an angle. Then you can lift it to the horizontal (so the track "lips" stop it from escaping) and attach a Tyronsea style deckplate and uni (about $90 from Windgenuity) or just a good old-fashioned style uni with the bolt screwing straight into the hole in the block.
If you don't want to mess around with that, get a normal waveboard deckplate or uni and a bunch of washers in increasing sizes to fit around the bottom screw/metal thread/bolt whatever. The smallest washer should be the normal size for the screw; then between than and the uni, fit a larger washer, then probably one or two more until you have ended up with a washer wide enough to only just fit into the slot in the mast track at an angle. Hold the whole bunch of washers up against the underside of the track lips with one hand, insert the bolt at the base of the uni or deck plate with the other, and then thread on a nut underneath the stack of washers and tighten it up.
I've got the same system (only two washers as the track is narrower) on one of my boards and it works well if you don't want to slide it up and down too much, which is not something you need to do all the time on a Runner.
The Runner is a good old board, and good old boards can be brilliant for kids as they are tough, they sail really well in light winds, and they keep upwind well. They are particularly good for kids who can already sail, as they have fewer balance problems and appreciate light-wind sailing , exploring and upwind performance.
Most of our kids and teens (about 4-6 at a couple of clubs, 10 or so at a couple of others) are on Windsurfer One Designs, with one on a Runner and a few on Raceboards or just starting on wide boards.
That's NOT to say that a short fat modern board is not better in lots of situations, because in many situations they are (ie if it's always choppy and windy). But the long old boards are also excellent in their own conditions.
The rig is the important thing - the lighter the better. Old rigs are generally pretty crappy due to cloth and layout etc; the Runner's stock sail isn't too bad for its age and perhaps lighter than many modern sails. However, it's big and long-boomed for learning on and the old wavesailors or pinheads many use are horrible.