In a piston aircraft, how much noise is generated by the engine, and how much by the propeller?
One particular example I can recall is Extra 330. Modification with electric engine have been built (Extra 330LE) keeping the rest of the aircraft quite similar to the original petrol-engine version as far as I know. According to this article electric version was measured to produce 14.5 dB less noise compared to the combustion engine aircraft.

Maybe you can try too look for other combustion engine aircraft converted to electric without making other significant modifications (I am not aware of any other than this Extra 330LE though) as a source for data you are looking for.