If it's just a cracked cover, then take the left one off.
It's a Skyline.... it can handle it
Too easy
Lots of options, starting with basic NA mods, extractors, pod, exhaust.
If he wants to go turbo, then the VL turbo bolt on bits will give him a nice and easy extra 30 to 50rwkw
Then if he's still power hungry, there are still places to go.
RB25det, RB26det, RB30det.........
How much does he want to spend on mods?
As far as enginge modifications go, its gotta be one of the easiest to modify.