I'm sorting through a deceased friend's aero-modelling effects and I found these Magnesium alloy castings for small Mills type engines. I believe that at least one is for a Mills 1.3.
The quality is quite good apart from a little corrosion. Much better than my crudely enhanced pic shows. He's also let a number of Magnesium Alloy speed pans which clean up very nicely even though they turn black again very quickly. One of these on a .60 sized c/l speed model pan is quite impressive. He replaced the original Aluminium alloy version with the Magnesium to save weight and to put the models weight under that allowed by a rule revision. Fifteen years later, it's aged very nicely with no sign of corrosion.
He did the projects at a local night course on metal casting at Newcastle Technical College. The Magnesium casting process was one of their specialities. They used discarded Volkswagon gearbox transaxle casting for metal, under the surface of some inert gas I believe.