Honestly, I think you should take your ski to the mechanic if you're not already proficient at working on internal combustion engines. I cringe saying that b/c I feel deep down that anyone can do this work but I've seen too many failed attempts, not being sritical, just making the suggestion. I've seen many mechanics screw them up and charge an arm and a leg too, which is why nobody touches my junk anymore(Well, they never did, LOL).
Anyway, to do this job you'll need both 5/16 and 1/4 inch inside diameter fuel line and 15 or 20 of the small stainless steel worm hose-clamps. I'm not sure for the ski, but the boat has I'd estimate about 12 feet of 5/16 and 6 feet of 1/4 inch on the carbs and return line.
You'll have to remove the carbs to change the fuel lines, so may as well rebuild them completely while they're off in order to avoid having to take them off again.
"it was parked about 3 or 4 seasons ago."
Yep, sounds about right. I wonder if someone properly mothballed it or it was rode hard and put up wet? If properly mothballed it's questionable as to the health of the fuel system but if put up wet, a no-brainer the fuel system must be gone through completely. This will require time, patience, and a good selection various tools.
How about the rest of the ski, do all the gauges work? The fuel sender repair could be done more easily if the fuel baffle needs to come out while the fuel lines are off of it. And yes, that fuel selector valve I understand isn't prohibitively expensive to replace unless you want to remove it and purge the trash out of it in all manor of directions with compressed air and cross your fingers by putting it back in. Just make sure it's clean and doesn't have any leaks that can allow air into your fuel lines, air leaking into your fuel lines as the fuel pump draws fuel from the tank is a big-time no-go, just as the gum and goo from those decayed and corroded gray Tempo fuel lines that fall apart and clog fuel filters and tiny carburetor passages leading to engine damage.
A two-stroke engine such as ours needs oil mixed with the fuel as lubricant, lack of fuel means lack of lubricant and leads to ring sticking to cylinder walls, bearings galling and pistons overheating. Don't run your ski out on the water if it's not running correctly hoping it might clear up on it's own, that's a recipe for engine damage. If the fuel has been sitting in that thing for more than a few months it has already begun decaying into a useless sour paint thinner and should not be run through a temperamental high-performance engine or any two-stroke engine for that matter, might get away with running it through an old car if it's mixed with enough fresh fuel and hasn't already turned into skunk urine. Otherwise if skunky, it's questionably suitable as Boy-Scout fluid.