The Spitfire and P40 use the exact same motor and gearbox as the P-51.. It's part number PKZ3624 for the motor and gearbox complete for all of the warbird ultra Micros, and the Sukhoi too. You can go to Parzone's web site and check them out if you like, the spare parts are all listed.
The two blade prop should help a bit but the real problem is that the new ultra micros have put on a lot of weight compared to the old ones. The old 'non-AS3X' UM's were about 35g, the new AS3X equiped ones are about 45g. That's close on 30% weight increase which will have a huge impact on performance on a model that didnt have a surplus of power in the first place. There really isn't much you can do about it other than try to get weight down by perhaps removing the landing gear and going to the lighter and more efficient two blade prop.