It really wouldn't matter, he'd be a martyr for "the cause" and everyone riding his coattails would be foaming at the mouth to replace him as the figurehead with big Trump posters behind em for the rest of time.
Tulsi, Vivek, RFK, NIki... There are a lot of choices out there and I think the Republicans would most likely do a quick round of debates/votes to get someone in.
...at least they listen to their party base. The DNC fucked over every candidate who wouldn't fall in line and bow. The base wanted Bernie, and look how that turned out. They don't gaf what their base wants, they'll prop up whoever they think they can control.
I'm sure the republican party would have another candidate immediately. There were a bunch of other candidates that ran a little bit earlier this year for this election, but got like zero news coverage and dropped out. I know because I looked them up a couple a weeks ago wondering why I never heard of others in passing.