Aloha PJ,
If the studs are original, the correct size is 10 X 1.5mm by 1 1/2 inches long. Dave DuBois and several other MG sites have a link to "XPAG engine fasteners" which is very helpful. I would suggest you check that sometime in the 65 plus years the studs were not replaced by the more commonly available 3/8 16 TPI studs.