Once you've got the right size screw, you can try a trick HO racers use when tapping the posts on one of those EXPENSIVE unused vintage AURORA, Atlas, or TYCO bodies. Apply just a small drop of Testor's, Tenax 7-R, or similar liquid cement for plastic models into the post. This will allow the screw to tap into the plastic with less resistance and decrease the chance of splitting or cracking. ALWAYS go slow when tapping a "virgin" post, it's time well spent. You can also sleeve the outside of the post with brass or aluminum tubing to strengthen it (this method is also used to repair badly cracked posts, too). Typically, the plastic used on ELDON bodies was a bit softer, making things a little easier. But - of course - BE SURE to test whatever glue you're going to use on an area inside the body, just to be sure it's compatible with the ELDON plastic.
Hope this helps you out a bit. Enjoy running those vintage ELDON racers! -- Ernie :>)