Nothing, take out the old gasoline. Refill with clear gasoline and oil (2/3%). Change the spark plug for a new one. Open air shaft. Kick to start it or push it in 2nd gear. You’ll see it rarely fails.
These things are built to last. I recently started one almost as old as yours without a problem.
It’s not been started in 13 years, I’m not sure anything I do now can save it.