It's getting a tad late to replace Trump, even if he steps aside for Romney, Nugent or someone else.
Unless Trumps voluntarily quits, it's too late to change RNC rules to replace him. (It would take a majority vote of the RNC's Standing Committee on Rules, followed by a three-quarter majority in the RNC. And it would only take effect 30 days later.)
Many Republicans have already cast their absentee votes in the key states of Florida and North Carolina.
The deadline has passed in many states for changing the names on ballot papers. That's not a show-stopper: Voters would be told to "punch Trump for Cruz."

When Mark Foley resigned in 2006 it was too late for the Republican Party to put Joe Negron on the ballot. But its "punch Foley for Joe" campaign failed and they lost what had been a safe Republican seat.
And it would come after the final humiliation of Cruz: Video of Cruz on the phone trying to convince voters to support Trump.