It looks like the fan is 120V and just evacuating air out of the chassis, not the most efficient use of a fan, but easy to design and build. You have plenty of room to work with. I believe a 4" or 5" pancake or vane-axial fan will work. For the rear of my amp, I wired (2) 120V fans in series. They ran half speed, but moved the same amount of air as one fan while being much quieter.
Your fan may also be temperature controlled, a nice feature, but it could mean the fan is a proprietary part. If that is the case, you can replace it easily with a full-time fan.