

I think if mothra was big enough , had her magical golden spray dust seen in the heisei mothra , and she some how could regenerate or have some sort of shield that she could beat king ghidorah

I have always viewed Mothra as the most magical and least constrained by what little "physics" are in these movies. So, while it doesn't appear physically like much match for Ghidorah, I think a plausible enough reason could be written for 1 Mothra to take out 1 Ghidorah.
Host of the podcast Giant Monster Messages where we watch EVERY giant monster film and look for the messages.