Don’t even waste your time bleeding it, most likely it will have all kinds of rust inside the pistons and calipers.
Your best bet is to take all the hydraulics apart, hone your cylinders and replace all seals. You can find cylinder cups from an auto parts from a rear wheel cylinder rebuild kit that will fit that diameter, and for the master cylinder seals you will have to contact a kart parts supplier, they can still get those old kart seals, just ask them.
And for your bleeding procedure, just buy a turkey baster, remove the plunger and use it as a funnel screw the tip into the threads and fill it up, jack up the front end of the kart put a wooden block underneath and start gravity bleeding your rear calipers. Then lower the kart down again, and start doing the front calipers.
Here’s a store that has your seals, contact them, or if you already know your size then just order them from their website.