EM007 MAC ENGINE

I would like to know what alloy and/or specification would have been used for the prewar MOV crankcases.
I have an MOV that has been raced since it was imported into Australia before the war. It was only laid up from when it became uncompetitive in modern racing classes in 1961 and when historic racing started here in 1975.
It has continually been upgraded and modified for more speed over the years and is quite competitive but I am now plagued by cracks in the timing side crankcase that refuse to be welded permanently.
My welding expert wishes to know the alloy composition so he can remove all the old repairs and weld it confidently. He recommends heat treatment to return it to the strength and durability that might have been lost due to poor welding practices and incorrect heating over the years.
So, can anyone help with this please, I want to return the old girl to the track in an oil-tight condition as soon as possible, she's getting twitchy. - Greg Summerton 29/2/2004