Hi Sonnytex,
I have used a 45 deg inverted setup. No problem at all. The exhaust exits between the undercarrage legs and if you fit an exhaust deflector (45 deg rubber fitting) this sends the exhaust gunge away from the model. Always good, I Hate cleaning airframes at the end of the day.
An upright 45 deg tends to be used on high wing trainers, to stop the exhaust gunge hitting the wing.

At the end of the day you can mount the engine at any angle to assist the installation, ie throttle linkage, tank height, cowl limitations.

There is no wrong, just remember, prop to the front.