RE: inverted four stroke
We've never used a McCoy plug, so we cannot tell you anything other than to try it and see what happens.
Usually, inverted running can be problematic with all glow engines at lower throttle settings. You usually wind up with a higher idle speed to keep the engine running, or you add an on-board glow plug battery that turns on at low RPM. Many times, just getting the idle set a bit leaner helps a lot, and then upping the idle RPM. You'd then choose a prop so that the model will be able to land at the idle setting you wind up with.
Also, please realize that many engines will get better the more they are run.