If you’re not willing to spend the money, there isn’t really anything you can do. From the parts you have now, a header is about all you’re missing for basic bolt-ons. But even with a header, you’d be lucky to gain 3-4 more hp. Realistically, the only thing really worth doing to a D-series engine is turbocharging it. Everything else will be a waste of money, just to gain a couple hp and some better throttle response. I know that’s not what you want to hear, but it’s the truth.