Complex sentence: Kyoji, who had been the life core, is in bad shape after the battle in the Guyana Highlands, and since he can't replace Kyoji due to his failing health, Master Asia plans to use Domon as the Devil Gundam's new life core.
Split into simpler sentences: Kyoji, who had been the life core, is in bad shape after the battle in the Guyana Highlands, and Master Asia's failing health makes him unable to replace him. Thus, ultimately, Master Asia plans to use Domon as the Devil Gundam's new life core.