High-mileage 915 J heads... can they be fixed?

That looks like a good option, I'll keep that in mind depending on how costly it'll be to fix the heads I have.

Although my lease at my new place will start in August there's no rush to move out of my current place since it's my parents house lol. But I don't want to wait longer than I have to.

It will cost between 400-8-- bucks to have those heads properly repaired....possibly more. They will need guides as well, I am sure. Guide and seat work is not cheap. Add up guides, seats, valves, springs, retainers and valve locks. Then the labor involved. It's going to get expensive pretty quickly.