Ok, I own a 55 s, and have looked into what to do. Bear in mind that you will never have a 200 BHP firebreathing monster. That's what Punto GTs are for.
For the 55s, you can do the following modifications...
GSR skimmed, pressure tested and ported head (~£350)
GSR reprofiled cam (~£90)
Supersprint manifold, intermediate pipe and backbox (~£600)
GSR stage 2 remap (£300, includes £109 GSR induction kit)
That should squeeze maybe another 20 hp out of it, so you will have 75 bhp. The 55s is the lightest, 'no-frills' of the bunch, so the real money should be spent on handling and braking mods for the tight-twisties.