Pump size question

Hi all,

My pool pump is going on 30 years old. This year the cover to the electrical hook up rusted away. The casing on the motor is pretty rusty and most of the paint is gone. The pump is a Stay-Rite P2R. I believe it is a 1.5 HP pump. I am thinking I should consider replacing it over the winter. My thinking is that, yes it still works but, considering it's age and condition I should be proactive.
That being said, I have a 3/4 HP Magnetek pump that was given to me. I'd like to save the money and use it, but I don't know if it is strong enough for my pool. My pool is 18X36 rectangular 25,000 gallons. Two skimmers plus a main drain. Two 1 1/2" supply lines, one at each end. Any advise is appreciated.