Partstree apparently has the C1U M29D to sell, but for $120 plus shipping I think a decent second-hand whipper snipper would cost considerably less.
The problem caused by over-tightened needles is that tuning becomes excessively sensitive - tiny adjustments cause major effects on air/fuel ratio. I can tell you what I would do in your situation, as a desperate alternative to throwing away the snipper - whether you do it, and whether you are happy with the results if you do, are matters for you, not for me.
First, I would grip the head of the mixture screw in the chuck of a bench drill, and spin it up. Then I would gently polish the taper at the needle end of the screw with some abrasive paper of at least 400 grit, with the aim of removing the graunch marks while retaining a straight line of taper at the original angle.
This is a rather unlovely procedure, but it will probably make the mixture screw at least work better than it does now. If the result was unsatisfactory, I would look around for a second-hand carburetor that will mount on the same studs and gasket as the Zama. The carburetor would preferably not be another Zama: a Walbro would be favourite.
