Try disabling any game genie codes you might have then reset. If you have no Game Genie codes enabled, try changing the region before resetting. If that doesn't work, try using programs that fix broken roms or broken shecksum. If that still doesn't work, try reinstalling Kega Fusion (maybe you could have also gotten fusion from the wrong website, try getting it from https://segaretro.org/Kega_Fusion) If none of this works, I can't help you, you either have bad ROMs or your computer doesn't support fusion (or some other technical reason which I don't know since I'm not and emulator expert), try using other emulators, such as Regen or BlastEm.
https://sonicresearch.org/community/index.php?threads/sonic-1-how-to-fix-slow-boot-processing-checksum-check.3650/ Here you go
I can't seem to know the context behind this topic can you elaborate more? I have seen some Red screens, due incompatibility of the emulator or the ROM, sometimes if you mess with the ROM header in the Disassembly, it also gives a red screen... Anyhow, I need a bit more context here
The red screen is a checksum check fail. The checksum has to be a certain value in correlation to the data. This was implemented to stop pirates from modifying game code. However, as we are essentially doing the same thing by editing a ROM when hacking, we can trigger it unintentionally. Editing the ROM's header to have the correct checksum is usually the correct solution, although when you're building from a disassembly you can just outright remove the check (as has been bought up already). Also to note is that some emulators allow you to bypass the checksum check and play the ROM regardless. While this is a solution of sorts, the checksum check is still there.
Another way: It's an outdated program, but it will work: Use ESEII (Esrael Sonic Editor 2) and use the built-in "CheckSum Check", it will work with many Sonic hacks to fix broken checksums and remove the red screen.