The "normal" ambient temp rating for Gast pumps, for example, is to "Operate at 32ºF - 104ºF (0ºC - 40ºC)." (from the user's manual). This gives some leeway as I am sure the pump gets hotter than 104ºF, but this is certainly what you would shoot for inside your well house (open air temp). Around my neck of the woods, it's common to see temps above 100°F in any enclosed structure, let alone on days above 100°F outside in open air (maybe 120+°F in the building). So, you want to keep the well house interior as cool as possible. This, in most warmer weather climates, means to ventilate the well house. If your well house gets above the recommended temps without the aeration pump, the pump will just add to the heat and you should put a fan through the wall (or roof) and a vent on the other side so that fresh air can be moved through it.
The key is the temps inside your well house on the warmest of days. e.g. - if your well house gets to the 100 degree mark and all you have is a fan blowing air around inside the well house...it's just moving the heated air around and it will continue to gain heat without an exchange of cooler fresh air. Now, if your well house stays around the 80 degree mark, a fan would certainly help the pump stay cooler so long as the heat from the aerator pump (and other pumps that might be in the well house) does not build up above the recommended ambient temp rating.
The size of the building (and it's max temps) will determine the size of ventilation fan you would need. Think of using a common bathroom vent fan or oven hood fan sized for your well house and consider putting it on a thermostat controlled switch so that it only runs above 80-90°F.
Should you find the need to ventilate the well house, you have another option. This is what I did to keep the fan size (and electric usage) to a minimum...I built a box that basically sits over the aerator pump with a duct connection on the top and a vented bottom. My small fan is hooked to the duct on the top and pulls air through the box and blows it outside the pump house. My pump house door is very loose fitting and that allows fresh air to be drawn into the building, up through the vented bottom, across the pump, through the fan, and back out of the building. A larger/hotter building would benefit from reversing the fan flow so that the pump is always seeing the cooler fresh air from outside the building. My pump house is so small that I did not think that benefit outweighed the possibility of drawing rain into the fan duct. This approach merely ventilates the pump with a small benefit to the building's interior temps.
The fan I used was from McMaster-Carr and came with a thermostat control - Part number #9191K32. My system has been running the warmer seasons for over 3 years now and so far so good.
https://www.mcmaster.com/catalog/127/718