If the Senate determines the trial is constitutional then they have an obligation to require the Chief Justice to preside over it. He is commanded to serve as presiding officer by Article 1, Section 3 and must do so even if it requires the Sergeant at Arms to haul him into the chamber and strap him in the presiding officer’s chair. He can make a motion that the procedure is unconstitutional and if the Senate sustains his motion, he doesn’t have to serve, otherwise he can resign or serve. There is no “I don’t want to” clause in the constitution.