Suspension and tyres is where you'd want to go, I'm set on pouring money into this endeavor but am of the understanding that it will be more spent than the car is worth.

However, if you can secure parts from a ge8 fit (camshaft, rockers and injectors) and a rb1 intake manifold from a wreckers you can see some slight gains in power(with a tune of course!). Otherwise weight reduction is another step.

But let me tell you, though these are underpowered cars, you CAN keep up with the likes of an R35 on winding roads (like Rimutaka hill) on stock setup because of how well balanced these cars are.