get a half decent backbox and if you're going to get a K&N airfilter, get a panel filter. Even a new replacement airfilter regains a few bhp which would be lost with all the crud on the old one from the last 10 years.
A set of new spark plugs and leads won't do any harm, nor will an engine flush (like redex) or a service...
I've been told the punto 75 exhaust manifold can add a couple of bhp and it's a direct replacement, less than £50 from a scrappy I would imagine.