I wouldnt listen to the above comments if you actually care about the car. I have had my g1 for 2 years and refuse to get rid of it.
Personally, I prefer older cars over newer cars simply because there is less to potentially go wrong.
But to answer your question. Yes. There is a lot you can do. You can get a hold of zc cams and drop them in(direct swap) You can get crempo cams, zc high comp cams, eagle rods, exhaust/intake. Even seen a d16a1 with 0 compression in a cylinder take a 200 shot of nitrous and keep going and seen them running 10lbs of boost without any kind of hone or anything and continue to go.
The d16a1 ISNT a power house though, its just durable as hell. Bottom line though, its still easily modified. People just dont like it cause its a d series and its harder to come by the parts.
You can do a search for g1 mod list and I went over a lot of the stuff that can be done to a g1