you will be told to leave the engine alone etc as its not worth it. ive been there done that, i got good gains for a 1.2 was it worth the money NO, was it worth doing probably not, if i could go back would i do it again YES.
same goes for my 172, numbers dont tell the full story i spent about 2k on my 172 on performance mods and gained 20bhp, on my evo ill gain about 150bhp+ for that lol was still worth doing on the 172 tho.
panel filter in the airbox, larger air feed from the airbox to front grill, custom decat (or home make one with an old cat, smash the internals out of it, good but not as good as a real bypass pipe), cat back system, then map it for 98ron fuel, you will probaly gain about 15bhp at the max when mapped with all they mods but it will transform the way it drives and responds