Just to be safe, I tapped the two small overflow adjuster holes out on the throttle shaft arms and experienced what you described. They tapped very easy and I was left with a not-so-tight feeling. Based on that I opted not to tap the idle adjustment screws. There is a fastenal near here. I've have almost zero luck on web searches for 6-40s. The hex caps are about the only thing long enough.

I put 'er all back together and put it on this evening. I have very little idle adjustment with the original screws but not so little that I can't run the motor. It's way too cold out to fiddle with it too much.

If I still need longer screws, I might tap it up to 8-32 giving me some solid metal to work with.