The spinner that is included with the P-51 PTS RTF package is a 2 1/4" 3-blade spinner. If you wish to convert your Mustang P-51 PTS RTF to a two-blade setup, I'd recommend trying an 11x6 Master Airscrew S2 Scimitar or APC Sport.

Master Airscrew makes a 10x7 3-blade prop that should work with your stock 3-blade spinner, however, and it should offer substantially better performance than the Evolution 10x4 3-blade prop that came with the plane. There's nothing wrong with 3-blade props generally speaking, just the Evolution trainer props that come with Hangar 9's RTF setups.