I still have my very first real bike, an '87 ST400.
The bike has been neglected by me but still rides fine. Most everything is original even the cables and housing. The only thing that isn't original are the saddle, pedals, handlebar tape, brake shoes and the Wheels, I switched over from 27" to 700c wheels after I was t-boned by a Harley and taco'ed my rear wheel.

My question is what should I do to it first besides replace the cables. Everything still works ok, despite never being taken in for a tune.

Is it worth the expense to upgrade the drivetrain or leave as is?