You can easily make Mecha in SF using the existing rules with just a few modifications. I would recommend starship mecha from levels 1-6, power armor mecha from level 7+, vehicle mecha from levels 9+, or back again to power armor mecha for levels 12+.
Design humanoid vehicles and starships. Give starships and vehicles melee attack options. Use the living starship monsters as ideas for weapon mounts and facing rules.
High level game play is probably going to be teams of power armored suited PCs fighting against colossal threats and that one naked star shaman throwing magic missiles at the big bad. IF that is the kind of campaign you want to run.