They respond well to the usual breathing mods.

Throttle bodies, stand alone management a good quality manifold and decat and finally cams.
This should get you north of 200bhp
But the mk2 is still a heavy car and the above engine would suit something much lighter. Besides by the time you have spent the 3.5k to do the above you may as well of bought an St225.

Don't waist your cash on induction kits and shiny manifolds on the likes of Ebay.