Hi,
Might be worth speaking to MGB Hive as i see they have various screws under their fixings section. I don't recall seeing them listed as separate items elsewhere. If you can remove one of the remaining ones , that would enable you to determine head profile, length, diameter , thread (UNF or UNC etc.). Most threads on the MGB are UNF although there are some notable exceptions.
No doubt breakers such as "Welsh MG" could probably assist.
Cheers,
Charles