I'm attempting to remove the hammer screws with sheared off heads using chemical erosion. I read on the interwebs that simmering in a saturated solution of alum will dissolve ferrous metals and leave non ferrous metals unmarked.
I obtained some alum from the local chemists.
I mixed this with 2 litres of hot water, this was nearly enough to cover the timing cover when placed in the vitreous enamel roasting dish I bought just for this occasion. A couple of mugs of water raised the level high enough to cover the hammer screws. The whole lot is currently simmering on a low heat.
I'm not sure if its doing anything but in the reports I've read it can take hours so I'll just wait and see.