You’re right on the spellings; I’m incorrect. My bad. As to what to do now: That depends on what is in the party rules for the State of Missouri GOP. Every State Party has provisions in the event of the death or withdrawal of a candidate before the General Election. Usually the State Committee chooses a replacement.
Don’t count on that; when Bob Toricelli dropped out of the NJ Senate campaign in 2002, the process to replace him with Frank Lautenberg was “extort a rule change out of the NJ Supreme Court”.
An equivalent option is not necessarily available to the MO GOP.