Reality check time.
At 17 I was the world's best driver. Insurance was unneccessary as I was too good to have a crash and it simply wasn't fair. I deserved a Ferrari with my talent and couldn't understand why the insurance company (and my dad) didn't agree.....
I'm 40 now. Still just as good mind
No seriously, putting a big engine into your Corsa, you are about as likely to wrap yourself around a lamp post or tree and anyone. statistics don't lie. Not only will your insurance company not pay up, but if (and when) you prang yourself, the police will charge you and you will have to start all over in a couple of years, zero no claims, etc, and a criminal record to show for it.
Try getting a quote for your Corsa as it is now, then have a think. Is it really worth that risk? You won't look cool when you are waiting at the bus stop, will you?
The Insurance company's assess risk so they can manage payouts. They have assessed you as a bloody danger not because they don't like you or are unfair, it's just your age is typical of the nutters out there.
Take a hint. If your conversion was a good one, get the most you can for that Corsa as soon as you can, and buy something STANDARD. Drive it about carefully and build no claims. Your No Claims discount is the route to nirvana, not red top engines and 4wd Scooby conversions.
Once you have full protected no Claims, THEN do you have fun. If you've saved funds too, you can buy something properly tasty, afford it and be proud of it.
Cheating and defrauding Insurance companies is not the route you should consider.
I ran a 1.3 Escort, with XR3i bumpers and spoiler to start with, then a 1.4 Escort done up to look like an RS turbo. Insurance was stiff, but the engines were stock and didn't affect my cool and posing ability. The Insurance didn't penalise me for the huge wheels and brakes either.
Now I can have what I want and even a full on Sti is only £350 fully comp, fully protected. Actually and weirdly, a Lambo Gallardo would cost me the same to insure.