Orange man is litteraly a russian asset and it should be enough to jail him…
But to answer on a theorical level you cannot prevent felons from politic in a democratic society, otherwise you just throw accusations at your opponent to win elections and sadly it happen in a lot of places
He hasn't been sentenced yet and won't be for months. Its possible that he won't be sentenced until after the election. He's also been indicted for paying the porn star, Stormy Daniels, hush money.
So, out-of-context, its just another rich dude doing money crime in the US. Its not like he's been convicted of being a russian asset or doing war crimes. Convention is to give him a slap on the wrist. But external political pressures and him being a unique dumbass to the judge will probably result in a tougher sentencing. But I doubt there will be any prison time, even white collar.
Also you cans still run for president from prison. Eugene V. Debs ran as the Socialist Party candidate while jailed for sedition during WWI.