Wow Moncha your brother-in-law really knows his stuff, impressive!
I don't remember it running hot but can see where he means (now you have pointed it out).
So a full rebuild is in order then, sounds expensive but fun, I do like tinkering although it would be nice to get to drive it more than 50 yards before I get yet another problem.

So what you are saying I need to do is

1. remove the other head
2. remove all 4 cams
3. remove all 24 valves
4. remove all guides
5. remove all seats
6. get both heads skimmed
7. buy new valves, guides, seats and headbolts
8. rebuild both heads
9. buy complete gasket kit
10. rebuild engine


Surly if I am going to go that far it would make sense to drop the bottom off the engine and replace the piston rings and main engine bearings.
******Then I would have a new engine******