Here are the base rom files for the original 1991 28-team version as well as the 2017 update to cxrom’s 32-Team edit. These are considered the “base” roms to apply any changes to.
T-Borg-20XX-Base-ROM.nes Tecmo-Super-Bowl-U.nes tpc_original.nesTecmo Super Bowl 28 and 32 Team Base Files