OK - let's see if this helps...
When I peeled the roof rack off of my 88, I found that it was held on with rubber well nuts. While the screws used a very odd thread, I was able to close the holes using, I believe, #10-24 machine screws and corresponding well nuts. Size the well nut, then get screws to fit (use a countersunk flat head, since that's what was there originally. The countersunk/punched depressions are already there, and you'll want to be able to slide the crossbars back and forth to adjust them. A little silicone on the well nut and screw threads won't go awry...)
The trailer hitch is usually held up with nut strips and screws to fit them - I believe they're a metric/ISO thread. See if you can source the nut strips at your local dealer, since that's the easiest way of getting them in there. Screws can be bought at any well-stocked hardware store, once you have them.
Most of the interior "tapping" screws are #6 SEMS screws in various lengths, usually 3/4" or 1". You should be able to find those at your local parts house - they're fairly common. You can also use a sheetmetal screw, but stop if it feels loose. Some may have been a #8 size.
Hope this helps!
