I think the general concensus would be if the bike is running fine and your doing normal riding, don't change anything. If you are concerned about the condition of it, first step would be to do a compression test by pulling plugs and putting guage in one head at a time. If it has decent compression then just ride it.

If you want to do a bit more do a leak down test to check the integrity of the crankshaft seals. That will require a bit more work and fabricating some stuff. I see you are in Spain so no point in me offering to send you my kit as I'm in US. Basically you pull pipes and carbs and plug them. I think oil injector needs plugging too, I forget. Mine is removed and permanently blocked. I use PVC pipe to plug the intakes and aluminum plates with rubber gaskets made from an old inner tube on the exhausht sides.. One of my plugs has a Schrader valve so I can pump up the engine. Do one side at a time. If no leaks your crankshaft seals are good. If leaking, you should plan to split case and replace them. Eventually the crank seals go bad. As you will learn, you can still buy the end seals but the center may be a challenge. The option then is to drop some bank and buy a new crankcase.

Of course if you get this far you're might as well replace all the crankcase seals, toss in new pistons, rings and whatever else your budget can accommodate. Then you will have a nice fresh motor with a known condition.

Good luck and keep us posted.