Here's a old engine tuners trick to try. The 20fp is a brass bushing-ed motor,you mentioned alot of in and out crank play. Remove the prop thrust washer and place 1 or 2 {depending on thickness } teflon shim washers between thrust washer and crankcase replace the prop of your choice and try running it again. I'm willing to bet it cures your troubles. It's always cured mine on the fp's.

happy flying