I am aware that your mag has been rebuilt, however, this does not rule out the possibility that something has gone wrong. I'm only suggesting this because the symptoms are like coil failure under load. This can happen if the coil has been wound with the windings too close to the armature so that, under load, the spark arcs onto the armature cheeks as this is an easier route than across the plug tips when the cylinder is full of compressed gas. Depending on the type of poor running you have there is a similar problem caused by capacitor failure (bike won't rev). Another reason I'm suggesting this is because it is a relatively easy fix and will instantly rule out magneto failure if the bike still performs poorly. Similarly, if you can borrow a carb, just swap the whole thing (carb and twistgrip) from a bike which runs OK.
