Hi hatman
I don’t know what 4HP Moffat Virtue used BUT other makes of engine where the head/water hopper is fitted the same way, brass dome nuts were used, this was done to protect the head stud so if ever you want to undo it you can and yes I have seen normal steel nuts used and if you want to undo them you can’t as they have rusted away along with the stud
I have also seen plain brass nuts fitted which makes it easy to fit on a spanner but it does not get you very far as the exposed head stud is all rusted up
Washer used? I have seen steel and copper used, copper would be best as it would compress a little bit which would give you a better seal
I saw one chap fit Stainless steel head studs and nuts so he could always undo it
It does not have to be original, who is going to see it as long as it works for you its your engine your money your time
Ian