Although a lot of people don't see the point, to me... the fun is in the work and the modification. Sure praising your work afterwards and driving is rewarding but not as rewarding as putting in the hard yards just to make it that much better!

Considering that you don't want to spend a lot of money... if you are mechanically capable, experienced with a Dremel and have a steady hand I've seen many people that have polished (some ported but you have to be careful as without approprate acompanying modifications and calculations you might actually make power/torque loss with porting) to their cylinder heads with good success. If you make a mistake the great thing is parts for a d-series engine are so cheap!!! =D You have fun and even if it's only a minor gain its bragging rights!!! haha.

I'm not familiar with D-Series though so I'm not exactly sure what their major bottlenecks are. I would assume they're similar to B-Series. (Excluding the obvious and typically Honda bottle neck of capacity.... lol).

I see you're organizing a Winton Track event... are you going to the upcoming one?