I don't even want to get into an alternate 1976 and how that would effect 1980 because I'm not a morning person.
If Reagan falls off his horse in 1978/79 however then the answer is almost certainly no-one. There's not the time for someone on the right to emerge with his stature. The vote there will be fragmented between Crane, Connolly and other lower-grade gadfly candidates. It's the predictable answer but it's most likely Bush who is the nominee in 1980. He had a strong campaign, particularly in Iowa, at a time in the early life of the primary/caucus system when strong campaigns or even competent ones weren't a given. (Look at how disastrous Dole's was) He wasn't gifted the runner-up status as people usually assume either, Howard Baker probably had more establishment support.