Running engine in reverse Harmfull?
Reed-valve engines run backwards with the same enthusiasm as forward. The old Cox Babe-Bee is an example.
One manufacturer of jet-skis has twiddled with the electronics to get reverse out of his jet-ski motor. These are also reed-valve motors, but have spark ignition. If you hit reverse, the electronics shut down the motor, then just as it stops, the spark plugs begin firing with revised timing to make the motor run in reverse rotation.
In 2-stroke with rotary valve such as the typical glo engine, nothing gets damaged from running backwards, but the rotary port timing makes sure that it runs crappy...it's just not getting enough air/fuel to run properly.