Of course it'll fit, but the better option is to sell the diesel Saxo complete- it'll have good resale as it's a small diesel, and then buy a VTR which will come with the VTR suspension, steering and brakes, and a petrol tank and petrol pipes and exhaust.
you could also squeeze a bit more go out of the existing engine by adjusting the fuelling but it'll smoke a lot and won't give you that much more go, but it'll help.
You can also squeeze in a HDi-it's been done- but it's a lot of work and it doesn't sound as if a diesel is what you actually want anyway....
Just buy what you really want...a VTR!!