Like a lot of things the answer as to whether Trump if a convicted felon can run for the presidency is that it depends. Some states will not allow someone to run if they're a felon or happen to be in jail at the time but this applies to state office. Some states will also allow someone to run but unable to hold office making the whole thing kind of pointless but that's at the state level. If a conviction puts Trump in prison then he'd unable to run for reelection pretty much on the basis that he'd be in prison and unable to leave in order to campaign. Trump is 74 according to Google and also according to Google the sentence for Tax crimes would be about 3-5 years. He could die or become unfit for duty during this period especially if convicted later and the sentence is put on top of that. At this current juncture within Biden's time in office he'd be unable to run purely on the basis he'd be in prison and be unable to serve. There's also the question of if a person in prison can perform the duties of the presidency and whatever crimes committed might bar a future President or if related to the former's time in office if that might disqualify one from the position. Like a lot of things Trump has done and did we don't have any easy answer.