The runner-up in the primary was John Brunner. (Sarah Steelman finished third.) Then there are two former senators available: Kit Bond and Jim Talent.
There are bound to be some procedures already in place. It’s not like someone dropping out is unprecedented or unthinkable.
At the least, the procedures in case someone unexpectedly passes away should apply.
I read that they have to file tomorrow, so the GOP better have somebody lined up.