I currently have a Hayward super pump 3/4 horse. It has gone to the land of pumps in the sky. I have access to a free Hayward power pump 1.5 horse from my uncles old AG pool. Will this work with my IG pool or do I need to buy a new pump?
Assuming that free pump will run - it will work, although it is a bit oversized for your application. I would check on on what model pump that is - some pumps specifically for above ground pools require that the pump to be below the water level of the skimmer. If the pump is an inground model (SuperPump) - no problem. Above ground pump (PowerFlo LX) - not so great. If you can get the model of the free pump - we can give you a better answer.
Hayward marketed that pump as an above ground pump (predecessor of the PowerFlo LX) - if the pump is below your water level - that should work OK - if your pump is above the pool - I'd get another one. Something like your dead Superpump (3/4 HP).
The difference here is that aboveground pumps have gravity to help prime them, while an inground pump is self-priming (well, after you fill the strainer to get the seals to work).