Hey Heyyy.....Trav me old mucker
Poor Nissan gone
Advise wise I think most would say just leave it alone

if you increase the boost you increase the problems mostly on that engine. Turbo's, head gaskets, cooling system, engine mounts etc etc.
If you want a cheap reliable car (in today's iffy climate) then leave it be
Eggsauce wise you'll be looking at £300 - 400 for a stainless jobby but why bother? I'd ring around the usual suspects for a standard steel system and then paint the hanger points yourself with high temp silver to help stop them rusting.