Strangely and coincidentally I am currently making a tool to do this as my original tool is now knacke...d! Lucas originally recommended a 'wheel operated screwdriver' which is a brace which wraps around the dynamo enabling a srewdriver tip to be screwed down onto the screw head then using a wheel or T bar the screws can be unscrewed. The problem with using an impact driver is that so much force is needed it is possible to knock the dynamo casing (referred to as the 'yoke' in Lucas literature) out of round unless the pole shoe is supported very solidly internally. By knocking the yoke out of round it is possible that the armature will come into contact with either the pole shoe or the yoke so it's best to avoid whacking the screws. If the screws are particularly stubborn I would be inclined to drill them out, as per tevinoz's suggestion, and replace them. Similarly, when rebuilding the unit it is crucial to avoid any airgap between the pole shoe and the inside of the yoke. Make sure the countersunk heads are a good fit in the yoke before commencing the rebuild and employ a pole shoe expander (small scissor jack like thing) to squeeze the pole shoe against the yoke. Ideally, use a wheel operated screwdriver to refit the screws.
