I'm not familiar with PPW Machima Motors but why are you trying to replace the Athearn motor? If it's just broken, you can pick up Athearn motors very cheaply on e-bay. There are also a number of other can-type motors that will fit most Athearns if you're looking for a motor with better performance.
Well I've been looking to do an Athearn repowering project and well this old Athearn blue box switcher could use a new motor. Athearn switchers sounded really bad back in the day and I was looking for my worst sounding athearn to repower and that one was it. Mashimas fit in I thought all athearns but I was somewhat iffy about a Baldwin switcher. I just thought that Mashimas were the best motors to put in Athearns to better their performance.
Is yours the BB version with or without flywheels? If it's the pre-flywheel version then repowering isn't going to make a great deal of difference without also upgrading the drive train. If it's the flywheel version, then replacing the motor with the one at http://www.accuratelighting.com/hoscale/Motors/HoscaleMotors_Style5.htm has worked for me. I've never found an Athearn BB locomotive it wouldn't fit, including the NW7, so I assume it would work in the Baldwin.