The only reason to mount any engine in any position is usually for scale or appearances. Glow engines tend to be a little harder to start if inverted. Four strokes are the exception to that rule. Side mounting will usually keep the profile you are after better than an upright mount. If a kit maker suggests a mount position it is usually because of appearance. The analogy of the P-51 is perfect. If you have a trainer type there is usually not an appearance problem and all model engines run better and start easier in an upright position.