Much like Bob Dole, Ike would have been too conservative to ever actually switch. At a stretch, he might have voted for McMullin in 2016, but I would really doubt that. A more interesting question is who he would have supported in GOP primaries.
1972: Nixon
1976: ??
1980: ??
1984: Reagan
1988: ??
1992: ??
1996: ??
2000: ??
2004: Bush
2008: Romney?
2012: Romney
2016: ??
2020: Trump
I'd guess something like:
1976: Reagan
1980: Reagan or GHW Bush
1984: Reagan
1988: Dole or GHW Bush
1992: GHW Bush
1996: Dole
2000: McCain
2004: GW Bush
2008: Romney or McCain
2012: Romney
2016: Kasich
2020: Trump [reluctantly]