Waow Sam, seriously, your on about keyboard warriors then your on preaching to people about how you wouldn't mind changing the engine and not declaring it.
Aside from the fact that your insurance will be void, what happens when the police pull you over and check out the car? Sounds retarded to me, I know a guy with a 2.0 in a 1.0 registered corsa and he's scared to drive it incase he gets caught, sounds great.
Btw, your 1k a year insurance quotes on the 205, named driver or what? Seems rather low for such a car for the first year, I smell some verbal. (Either that or your quotes are for before you've passed your test)
On the note of not having to declare everything, your insurance company is here to make money, so they will do everything in their power to find a reason to not pay out if you have an accident.