I have made a small tool to fix the internal GameBoy ROM Header. It's a simple single source file compiled with g++ (or any other c++ compiler).
You can find the tool here: https://github.com/elseyf/gbfix
[Tool] gbfix, simple cmd-line tool to fix GameBoy Headers
Re: [Tool] gbfix, simple cmd-line tool to fix GameBoy Header
It is very similar to RGBFIX.