amend the current Presidential Succession Act

The order of succession is not in the Constitution, so all that is needed is to have Congress amend the current Presidential Succession Act - probably in response to voter outrage or some such - so that it includes a special election the December following the emergency. The original Presidential Succession Act of 1792 Section 10 required there be a presidential election in the December of the year in which section 9 was invoked (or the December of the next year, if there was less than two months to go until December and the presidential term was not about to expire). That Act has since been changed and is not in effect (the current one is 3 U.S.C. § 19) but if it was restored and the President elected in 2016 was incapacitated or killed in some way in late 2017 then there would be a special election in 2018.