Thanks for the responses guys! Repacking the silencer sounds like a good idea, but, - I'm not sure exactly how to go about that. Does that "biturbo" come apart? and if so, is there a particular material or packing that is available at the local moped shop?
AS for the speedometer, I never gave it a thought that the erraticness might be in the cable and "sender", and I have had many apart before. I'll get on that right away!
As for the jerky deceleration, I never considered that the 2nd gear clutch may be the problem I have another Tomos (94 Targa ST) which doesn't have the jerky downshift, but I believe the shift to 2nd gear is at a much higher speed. I have seen articles about the different "clutch springs" that provide different speeds at which the clutch shifts. So looks like I may have to experiment with different clutch springs and try to get a shift at a higher speed.
Thanks again for the interesting responses!