To run mods like the LML (Lenny's Mod Loader), you will need a compatible version.dll and the correct Script Hook for build 1436.
Ultimate Edition bonuses (Special Edition outfits, Thoroughbred horse, gameplay boosts, and free access to additional weapons).
If you experience crashes on startup, try deleting the .sga files in your Documents\Rockstar Games\Red Dead Redemption 2\Settings folder or switching from Vulkan to DirectX 12 in the game settings.
If you are looking to install mods or find your specific save files for this version, these tutorials cover the process step-by-step: