Why do some designs call for inverted engines? Is it a performance issue or an aesthetic issue?

as far as I can tell, it's pure aesthetics. Mainly to clean up the lines that are visible above the cowl--to look more scale. It isn't performance. In fact, care must be taken when inverting an engine not to flood it. Once running, they pretty much run the same as normal-mounted engines, but some need onboard glow to help with the idle since the fuel can more easily get onto the glow plug and cool it off.

I must say that I have been very fortunate with running my engines inverted. I have run 2C and 4C inverted without any problems but for the first start of the day. Even that is sorted out by using an electric starter. What I have found to be helpful is a refueling valve, this ensures that the fuel does not over flow the carb when you are filling up. Taking some time to patiently tune the engine helps a lot. Guess, for me its listening to the manner in which the engine runs and throttles up.

One thing I hate is when we are not using a remote glow ... hate trying to fiddle to get the glow in!

I have a few planes with inverted two strokes but I also lower the fuel tank to accomodate the change, If you don't lower the tank,the fuelsiphons down and can cause tuning problems and your just begging for problems. Some don't lower the tanks at all and whatever but by lowering the tank, I have no related issues on it all.

Another reason for an inverted tank is lateral CG. While smaller engines dont have a big effect on that when they are installed sideways due to a lighter smaller head. Bigger Glows YS's 140's and 160's can get quite heavy at the head. And whoile engine torque may take some of that away lateral balancing sometimes sufferes. Pattern guys are notorious for installing the engines inverted my guess that is one of the main reasons.

if you have a clasic WWII plane with the ducting in the lower part of the cowl for airflow it will allow the header to be in more direct airflow cooling the engine better.. that and its easier to port the exaust in a scale plane if its inverted since the mufler wont be sitting clear on top of the fuse. but there right it doesent do much except leak fule if your not carful

So far i've only had 3 planes, and one that is inverted, my 46AX in the WM Dago Red, and it hasn't been an easy intro to inverts. Its hard to get the tune to stay consistant and correct, but i guess that will come with experience. The only thing to note is that a coweled engine needs to be run richer, to prevent heat damage. The reason i think for my stalling problems during flight is because the tank is too high, but due to the way the wing mounts and the amount of room for the tank it isn't possible to modify it and lower the tank, so its just gunna be a persaverance issue i think until it goes dead to far away