What should be my concerns when replacing this timer? 120v Diehl type 880 24hr there are contact points numbered 1-5 with connector tabs at 1,2 3 & 5

Be sure the clock motor is the same voltage. Yours is as simple as it gets, so you might end up with one that's got more uses than you need but you'll be abe to use it. You can also consider changing to a 7 day clock vs 24hr if you like.