You are correct, Hitec seems to think it's a brilliant idea to use screws that you can't buy anywhere! It is a 2.3mm or even a 2/56 screw - I have tried both and they both work fine. You'll struggle to find them though, I know Makita drills use 2.3mm internal screws. I got a pack of 2/56 screws from
www.amazon.com (yes, I know, Amazon!)
The 3Racing pin on mine was tight but it did fit, I guess your best option would be to grind it down a tiny bit to reduce the diameter.
Get some pics up when you are done bud.