Pretty impossible to say ... are we to assume he came up in today's politics? Are we to assume he's immortal and has lived through everything since he left office? Since this is a wild exercise to begin with, I will be safe and say that he would, as a former Democratic-Republican, likely be a Democrat. But that guess literally is based off of nothing.
These are important questions. If we assume they came back today with the same views as back then, the founders and most Americans alive before WW2 would be white nationalists. The capital of Liberia is literally named after Monroe because of his efforts to promote it. The entire goal of Liberia was to repatriate the free black population. Even Lincoln wanted to send freed blacks to Central America. The kinds of mass deportations early presidents supported go well beyond anything Richard Spencer has advocated (to my knowledge, he's said some ridiculous stuff before).
Perhaps if we disregard this aspect, we'd find that Monroe and the others would mainly be concerned about the size of government and its trampling of individual liberties. since both parties do this, it's still hard to say. Perhaps he'd form a paleolibertarian third party.
If we assume they were raised in the society of today, then they'd just be ordinary people whose views might not resemble those they would have held if born in a different era, making the question sort of a moot point.
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