There's plenty which can be done to free up more horses, but it all begs the question - if you want a (relatively) powerful 500, why not buy an Abarth which is powerful out-of-the-box and therefore less likely to fight your attempts to squeeze more horses/torque out of it?
If you don't have air-condition a Rotrex C15-20 Supercharger is a possibility.
It will give you about 110-120 hp by 10 Psi.
I have most of the parts boxed-up.