Is there a mod out there or a file I could modify so the crashed planes stay on the ground the time of the mission?
I think they disappear too fast, I know it's just a detail but I'm all about immersion and realism and I never liked disappearing objects in games/simulators.
This isn't something that a player should be allowed to control.
If you build your own maps then you may set each vehicle's 'Delete after death' flag. If this parameter is false then the vehicle does not vanish, although currently it does not work for locomotives which disappear quite soon, whereas orphaned rolling stock takes about ten minutes to evaporate.
In mission building it is good practice actively to delete a destroyed vehicle prior to re-spawning it, otherwise situations could arise where for instance an AI plane gets destroyed on an airfield then its successor spawns into the wreckage.