When you said the engine was "overrevving", what RPM were you running.  If you're running below the mid-12,000's there is no way the engine was over-revved.  If you prop the engine for between 11,000 and 13,000 RPM when the engine is leaned to just rich of peak RPM at full throttle, you'll be OK, no matter what propeller you're using.  However, the engine will be quite noisy.  O.S. suggests rather large propellers to give a low noise profile because many RC fields in Japan have very strict noise regulations.

Any model engine will become hot enough to cause a severe burn if you touch the cylinder and cylinder head while it's running or just after it's been stopped.  It will take several minutes for the engine to become cool enough to touch after stopping.

If the engine will sag in RPM when the aircraft is pitched to the vertical, you're going lean.  You have to correct the fuel-flow difficulty.  If the engine has an actual problem, it's best diagnosed on a test fixture.  There, you can put everything in the open and see how things run.

For cooling, you need to m ake sure that any air entering the cowling actually passed through the fins of the cylinder and cylinder head.  If you can see the air outlet when you look into the air inlet, you can be assured that the air is bypassing the cyilnder.  The air outlet should be at least 2 times large than the air inlet.  In many airplanes, some kind of baffle is needed to direct airflow through the cylinder fins.

The oil mixture should be 18% by volume of the fuel, and the Klotz 80/20 synthetic/castor is a good oil.

Finally, the exact number of turns on the needle valve when you set your engine is completely irrelevant.  The engine tunes wherever it tunes.  However, changes in the weather from day-to-day can cause significant changes in the needle setting.  Even flying from a cool morning to a  hot afternoon will require noticeable changes in the needle setting.  The needle needs to be double-checked before every flight, even if you only perform the "pinch" test or lift the nose of the model to the vertical.

On the .75, you can try 11 x 8 or 9 props, 12 x 6, 7, or 8, even a 13 x 4 or 5 if you want to hover.  Keep peak RPM below 13K and you should be fine.