The most important things are ..
Physical size in mm . Location of any mounting holes, the voltage used to drive it. If you have access to a multimeter that is easy to determine.. the number of blades ideally will be the same as the airflow is determined by the area of the fan, the speed of rotation and the number of blades. Then look in the CPC farnell catalog.
From what I can see on Google images, it's in a circular housing that clips into the rear panel. I suspect it's almost certainly proprietary. But there also appears to be plenty of space inside to mount a box fan & drill mounting holes in the backplate. Here's another source of fans.
But interestingly, according to that service manual, the fan was only fitted to models intended for GB & Switzerland. The rest of Europe didn't get a fan. Now given that Europe operates on the same voltage I can't think why specifically GB & Swiss models only would require a fan. Anyone else?
So as long as you have decent circulation around the amp it may not be necessary & can simply be disconnected. Obviously at your own risk & you need to monitor how hot the case gets.