If it will fit and if it has the correct size drive shaft, the Kato motor would be an excellent choice because they run very smoothly.
I've never used a Kato motor to replace one in a different brand, but all of the Kato HO scale models I own are quiet and powerful, and draw very little current.
I think you may find that your Athearn locomotive runs slower with the Kato motor because Kato in general seems to run slower than other brands.
I need to remotor an older Athearn Blue Box locomotive that just draws too much power for DCC operation. Even with a thorough cleaning of the entire chassis and all electrical connections (including the motor brushes), the original motor just sucks up power. I was looking at the Kato replacement can motor the other day and it looked like I might be able to use it to repower this locomotive. Anyone use this motor and what kind of experiences have you had with it?
Hornblower