Hi
I would suggest if this is an engine you want to be good and reliable, that this is the time to bite the bullet and spend some money to get it into the best possible operating condition.
You can replace the oiling system with new components so it will operate like new and be serviceable for many years or you could remove the oil injection system completely, and then mix your fuel with a high quality TCW-3 2 stroke oil.
Given the age of the engine one or the other will be necessary.
Before spending any money you should do a compression test to confirm that the engine is in good condition internally. As Crowman mentioned overheating is bad news, so it is most important to replace the water pump (I recommend every 2 years on these engines) check/test/replace the poppet valve and thermostat, cooling on these is critical and you don't want to destroy the engine due to overheating. Other items include possibly cleaning and rebuilding carburetors, replace fuel filter, install new spark plugs, replace the fuel lines with new ethanol compliant lines, change the lower unit oil and pressure test, check the condition of the plug wires and replace if needed, inspect all wiring for exposed terminals, chaffing etc. Make sure you have a good battery. It is very important to make sure that your RPM are in the proper range at WOT with a good quality prop in good condition. The boat should have a normal load in it for testing purposes. I believe you want the WOT RPM to be between 5000 and 5500 RPM. If you are not comfortable working on an engine like this then you should take it to good reputable mechanic. Hope this helps.