I can do some script mods, placing vehicles & weapons & have figured out how to set when they spawn if mission related. 
Have also done minor .TXD edits, exported them from GTA3.IMG & pulled the texture maps.  Have also found dance score, initial money & dealer cash in GTASA. 
For data files, have good luck w/ HANDLING.CFG, CARCOLS.DAT, CARGRP.DAT & others. 
AW NUTS, I just got a job, didn't I.