Remotoring Athearns is pretty easy. I've done a couple of geeps, several switchers, and some U-boats.
I use either Mashima or Sagami can motors: I got my Sagami through NorthWest Short Line, and the Mashimas at the LHS. If you intend to do any amount of remotorings, the NWSL catalogue is well worth the cost. Not only does it contain listing for motors, gears and driveshafts, but there's also lots of tips on doing remotoring jobs for various locos. Their motor listings contain spec sheets for all of their motors, giving all of the motor's dimensions, along with its rpm and horsepower.

In general, pick the slowest running motor that will fit into the space available in your loco. In most cases, it's better to go with a moderately-sized motor, then add some extra weight in the left-over empty space. Most of these motors can handle far more weight than you could ever be able to cram into the shell.
Ideally, the drive train should be as straight as possible, so you may have to modify the frame in order for the motor to sit lower. Another alternative is to use a flat-sided can motor, and mount the motor on-the-flat.
Different motors may have different shaft diameters, and none of them may match the driveshafts in your loco. NWSL offers brass bushings in various lengths, and various inside and outside diameters, and these can be used to match the motor's shafts to the drive train of almost any model.
I don't have an awful lot of motor pictures, but here's a Mashima flat can motor mounted in an Athearn switcher.
If I recall correctly, I had to remove only a minor amount of material from the underframe in order to allow the motor to sit level. The motor is held in place with silicone caulk. Those adapter bushings are a press fit on the motor shafts, then the Athearn drive component presses onto that. I believe that I may have had to shorten the drive shafts a bit, easily done with a utility knife. I also omitted the flywheels, as I don't feel that their small mass has much effect on running characteristics.
You can see the brass adapter bushing in this view, between the main body and the ball of the male drive shaft component.
Here's one of the U-boats - the motors sit almost too low.
While these locos are extremely powerful, I believe that they might have been even moreso with only one motor, but more weight. Total loco weight is about 33 ounces.
My friend Deano has the U-boats, as they were too modern for my '30s-era layout. And while the switchers are also too modern, I occasionally run '50s-era, and these locos run extremely well, either with one another or teamed up with any of my steam locos.
Wayne