considering all things, I think money would be better spent "adding lightness" compared to adding power. A number of things such as drivetrain/chassis would be hard to replaced, but a good portion of the body paneling are what make this such a porker. Lose poundage there and increase performance immediately along with lessening wear on components. Considering how little the S&S motor and the MX5 gearbox weigh, it should be easy to narrow down the portions where you could add lightness. Carbon Bonnet/Boot for starters?
Adding power will only add to the wear on components. Albeit, adding power is often the cheapest option from the gate. Though, the complications it can cause can result in it being far more expensive.
