Superchargers all generally all about torque which is what the Alfa gearbox isn't too good at. If you fit one, you need a fair bit of boost to offset the weight of kit fitted compared to the weight of the vehicle otherwise you can have even less power to weight. Only kit available is the rotrex but that's a belt driven turbo, more about top end. The right part for the job is an opcon/autorotor lysholm supercharger but you'll have a Merry time engineering it all . Sensible advice is to aim for 300 HP, ample in such a short car, you can achieve that with a high stressed Alfa installation or go the Toyota route which is what most here would now do if they were starting again.