Easy mods wont get you to 200bhp. most 172/cups start life around the 166 mark. exhaust/IK/chip may get you an extra 10-12hp, i.e youll end up with about 180 or so if youre lucky. Some people on the forum claim 185 - Maybe on a cool day, a happy rolling road and an engine with 10k+ miles. to get to 200, you will need a combination of head flowed/ported, cams, or throttle bodies, and these are not exactly easy mods (as in you cant do them yourself), and will cost you about £1k pr extra 10bhp. Head and cams will get you approx 200, add TBs and you should get to 220. Obviously you will totally invalidate your warranty doing that, and as the engine is already finely tuned you may have to be patient and willing to accept the odd mechanical problem.
If you want a faster car, I would suggest concentrating first on weight saving, suspension, and some track day experience