Welcome to the community.
I don't believe that the 770 had a ROM chip replacement. Unfortunately it would be a physical chip swap rather than a software/firmware change. We don't have it down as upgradable in the D&S database
https://www.hpcfactor.com/qlink/?linkID=9
If it will take a 790 CE 3 ROM, I'm sure Rich will be along soon to let us know; he's the MP guru.
Any patches that we do have for the 770, you will find in H/PC:Update -
https://www.hpcfactor.com/qlink/?linkID=176