With my shiny new impact driver I have finally been able to remove the screws from the magneto and extract the rotor. It was quite rusty inside and would not turn freely. I managed to clean the insides and apply some grease to the bearings. Once reassembled it now turns easily by hand and even produces a tiny spark! I have ordered a new set of contact breaker points and a new condenser for it. Hopefully I can manage to repair the magneto myself, as it is very expensive to have them professionally rebuilt especially if the coil is knackered and needs rewinding.