Sounds like you have a grip on what's going on. Good!!!
Just to cover the bases, these are general:
Main jet - from about 2/3rds throttle up
Needle - about 1/3 to 2/3rds
pilot - down low.
If you can't control the idle, then the pilot jet's too large. Seems like most run closer to a 36 or 38, from what I recall.
The air screw should make the engine start to rev up if you turn it out and blubber when you turn it in. If you can't get any reaction that points to the pilot jet. Your description says it's fat.
Once you get the bottom end controlled, start messing with the top end. Work from rich to the correct jetting. Two bangers don't like being lean at all.
Do the good ol' plug chop and work to a pancake brown on the electrode. You want to start out with a fresh plug. If you're really rich at first and end up with a sooty plug, put in a new plug with the next jetting change.
Once you find the correct jetting, stick the sooty plug back in and run it - that will get rid of the black stuff and you'll have a spare plug to carry around with you, "just in case".
The little c-clip - just a regular piece. Just make sure it fits well and doesn't flop around. It's a metric piece more than likely, but I'll bet you can find a replacement at the local hardware store or auto parts place.
For some more info, give this a read:
http://www.yellowdogracing.com/techstuff.htm