Hi mate , this is what you call dropping valves. Looks as though you are up for one piston , maybe a conrod if it has distorted , a set of mains and big end bearings because you will have the crank out, a quick crank polish and measure , if bore is lightly damaged a light hone or a bore out and then you will be up for a set of pistons and expensive machine work of boring all 6 cylinders. New gasket set complete .The head needs 4 new valves, guide work( i am not familiar with this engine so I do not know if head has guides.If so you might get away with slipping 4 new ones in. If you have not then head will be machined to accept guides. I could go on more but when you start something like this the more you go into it the more there is to fix if you want it done properly .Most important with this repair is to fix the CAUSE.
IM a mechanic with no Skyline experience but this engine works exactly the same as whats in Cat dump truck to Porsche 930. I hope I could help with some of the jargon.