I think the GOP is out of it now. Granted, they have a connection to the electors, but I think we fall into state laws at this point. Some states say you can't not be a faithless elector and must vote for the candidate. Meanwhile states may (or may not) have laws regarding cases of death of a candidate.
...and the new candidate would appear on the ballot on election day,
Ballot deadlines have passed, most likely.
if not sooner on mailed ballots as well. If he dies after the polls have closed, but before the votes are counted that same process applies, and the electors will still be the ones who ultimately choose the winner - though the concept of faithless elector gets a bit murky here. If he dies after the resolution of the election, then the standard rules of succession would apply even though he had not taken his oath and actually started his term.
There are specific dates for the submittal of ev's and counting of them, so those are likely critical.
This is certainly a muddled situation, if there is a death. Nothing like this has happened before, but it being 2020 and all... looks like the Mayans were 8 years off on their calendar.
Hopefully Biden wins out right and it doesn't matter if Trump lives or dies.