there are some backyard mods that can be done, but some cost money, there is no need for magna injectors because there will be no big power difference on just bolt ons, it will have to push alot more power or be a boosted car.
Start with the basics.
2.5 exhaust / extractors
Cold air intake
Diff gears 3.7-3.9s mini spool
take all the weight out of it spare tire jack all that crap all depends how bad you want it, you can go to more extremes like taking the deadner out ditching aircon there are alot of small things that can be done.
if he has abit of money get the heads shaved $100-$120 for both 30-40thou if you have a die grinder and bits and good with your hands port the heads,
a custom grind cam is about $200-$250 the list goes on. good luck there are alot of more things that i havnt said but if you do a search it will come up with tons of Qs and answers. but also at the end of the day its still a 6 and just with bolt ons isnt going to take on a gen III or something
yeah mate dont think he ment that in an offencive way but just start with the basics, do all the bolt on's and hunt down some diff gears. maybe do some weight reduction? try and set up a CAI, you dont need to buy it make one save some money and who doesnt like making ****? he'll like that probably. and good work for helping out your little brother
your bes tbang for buck will be in the diff gears. no it doesn not gain u any hp. but it will liven the thing up, hunt down some 3.89, or 3.7 diff gears from the wreckers out of nissan r31 skyline and pintaras, have them fitted for between the 200-500 mark. im sure that will put a smile on his face.