1969 BSA royal star problem

i bought this bike and it ran like a champ until i took it to work one morning about 2 weeks ago...........on the highway. since then, 3 things have happened. the voltage meter that is built into the headlamp came off (small problem), the speedometer stopped working (slightly bigger problem), and there is a "puttering" coming out of the left muffler. single amal carb. no puttering when i accelerate, only when i am cruising at a constant speed. it is a fast puttering. i dismantled the speedo and found that the square metal tenon that is inserted into the back doesn't spin when i'm riding, which must mean the part that is attached to the rear wheel is broken. any suggestions anyone?