Well many months later I got back to working on this one :P
This is what it looks like in the game so far:
I still need to add some more lines between the body and the hood, trunk and bumpers. The car looks way too reflective, I think it's because it's so low poly. I added a few more vertices to some parts, and added those details on the bumpers. I'm making it into the R/T version because it looks more sporty and more like something a street racer would drive. I don't know what version the one from Underground was, maybe the base one.
For the interior I'll try to adjust the one from the Integra from Pro Street, with some modifications to look like the Neon interior. Thanks for the body kit suggestions, I'll check those out.