Not sure if you found the fixes for these yet, but,
-The flush fitting can be bought on EBay, check out this link:
TIGERSHARK 94-99 JET SKI FLUSH KIT | eBay
-Not sure if you need to replace anything yet, did you check compression? The engine may just need to warm up and get the air out, make sure you have the garden hose hooked up too for cooling the motor.
-You can tell if the oil injection is working if the exhaust is smoking, other than that you would need to pull an oil line while it is running or maybe better during turning the motor over with the plug wires off. If you start the engine and it is not smoking at all after a minute or two, immediately stop the engine and make sure there is oil in the tank and that the oil injection is working. If it isn't you can bypass the oil injection by making a 50:1 premix and using that. (may have to empty the gas tank though)
-Another thing, if the engine has never been started or not run in a year or more it is always good to prime the engine by putting an ounce of 2 cycle oil or just less in each spark plug hole too. This will help with the engine break in.
-You can still get a lot of parts for these skis, especially on EBay and Country cat overstock.
Hope this helps.