How many km on this engine? A decently maintained Brick engine should run at least 200,000 km before it needs any serious mechanical attention beyond routine maintenance.
Run a compression test on the engine before you make any plans about doing a head job. A couple ml of oil in the cylinders and then crank the engine with the throttle wide open and all the spark plugs out. A good engine will run in the vicinity of 9-10 bar or even higher.
I would just check the valve clearances and try to get the exhaust valves as close to the wide end of the gap specification as you can. Intake valves work best at the narrower gaps.
As far as a tune up, make sure the air filter isn't too dirty, put in some new spark plugs(if you use NGK, make sure they have terminal nuts to make good contact with the spark plug wires), check the ignition timing, balance the throttle bodies and run some injector cleaner through the system.
If you can run the engine from time to time, winter is a good opportunity to clean up the injectors. Put some injector cleaner in the tank with fresh fuel. Then once a week during the winter, run the engine at 2000rpm for 5 minutes or until it reaches normal operating temperature. If the injectors are varnished with old fuel, this will clean it out and correct any stickiness in their operation. This is important if the engine has been idle for an extended period of time.
There is a reason these engines are called Bricks beyond the way they look.
