If amp get hot laying normally... putting it on it's side will probably be a bad idea.
I had some (long ago, in a galaxy far away) early Carver amps that always were cool to the touch, and had those puppies upended and no problems in several years of use.
If the amp get hot, I would not even try it, even with a fan! A fan taht blows enough air is going to be too damn noisy to use, and one that is quiet is not going to blow wnough air to work.
If the amp is hot to the touch (you can keep your fingers touching the hottest part, but it is uncomfortable) then it it TOO HOT to turn on it's side.
If it is just warm to the touch, at it's hottest externale part, you could probably up-end it and be ok.