buy another inexpensive prop-a Zinger Pro is fine -just get a prop with matching grain on each blade if possible .
the Zinger 6"pitch is a paint stirrer --try a 22x8 -then if it needs mor revs start clipping back the tips at an angle smoothingthem and balancing them a 1/4" at a time
see what happens - sIt should work if the engine is in goodrepair.
You simply can not get one of these to run too fast so let it rev .7000 on the ground is fine -
Your engine is made up from bits n pieces of industrial engine that was designed to run fast .
Your "muffler may be choking the engine - remove it n run it up and see.