i was going to do the conversion and not tell insurance
Doable but wouldn't advise of it... Call me a bore if you like but personally I'd buy an official 172, yeah the insurance will be more expensive than a 1.2 but if it's what you want you might as well go legit, if it's not necessarily the power from the 172 you want you could just put a 172 kit on yours? Make it a replica
Another option, would be if you love 172s that much, buy one that needs work, do it up while your driving your 1.2 and get some ncd behind you
As for the sunroof, I'd tiger seal it, or if you want to be able to use it still have it done properly, I had mine done on my valver, never leaked since either