[Jamesp]

I would not put any permanent magnet inside the engine. Electromagnet would be ok. A rare earth magnet will stick to anything ferrous and not come off. You will lose it in there and then have an even bigger problem. I'd get stronger magnets to use from the outside of the case to drag the screw up out of the hole. If you can pull the screw up high enough to touch the screw with the end of a coat hanger while both are in the magnetic field it will tend to stick to the coat hanger. As mentioned earlier the screw will be attracted to the IMS and chain as well as the magnets while both are in a magnetic field so you can expect to have to wiggle the magnet around a fair bit to tease out the screw. Away from ferrous material you should be able to move the screw around at will.