If the injectors work, no need to replace them.
Add 4 oz of Seafoam to a full tank of gas, this will help to keep them clean.
If you look at the ECU box, on the right edge facing the engine, you will see a port with an LED behind it.
When you turn on the key, this should flash once as it completes it's self diagnostic.
If it flashes more than once, count the number and duration as that is a trouble code.
It will tell you what component may have a short or bad connection.
If you look at where the throttle cable connects to the fuel rail, you will see a small screw. That is the throttle stop, turn it in to raise the idle rpm to about 1800.
You will have to remove the battery, exhaust, and silencer to get to the chain case.
If you are going to drain it, you should also open it and inspect the chain.
There is a drain plug inside the tunnel just below the drive shaft and the filler is under the battery. It takes 6oz of oil. You can use ATF.