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
1980 : Reagan (he was from the conservative wing of the GOP).
1992 : Bush (i feel like Ike would back the incumbent after he wins).
1996 : Buchanan