Athearn used to offer pretty much all of the parts for their locos separately - I powered some dummy units with can motors, and got replacement trucks with gears and sideframes, plus the driveline universals, although I don't know if Horizon still offers the parts in this manner. Couldn't hurt to ask.

You should be able to find used Athearn Blue Box locos, both powered and dummy, at a very reasonable price. To make a dummy from a powered unit, remove and discard the steel strap that runs between the trucks and the motor, then remove the driveshafts that connect the motor to the trucks and remove the worm from the trucks. To do this, use a small screwdriver to pop the cover off the top of both trucks. Merely lift out the worm and bearings, then pull the drive line away from the motor. You can simply remove the parts that slide away freely, or pull the entire works from the motor shaft. Snap the covers back into place on the trucks, as they help hold everything together and also hold the truck in place on the frame. Leave the motor in place to give the unit some weight, or simply grasp it firmly and pop it out. This modified unit will offer some resistance to rolling because there are still gears inside the truck - They can be removed, while you've still got that top cover off the truck, by also carefully removing the bottom cover. Use the small screwdriver, but be careful to not break the retaining lugs. On most Athearn trucks, there's also small third cover near the end of the gear housing. Separate the two halves of the casing, lift out the gears and reassemble everything.
Don't remove the gears that are part of the axles. The original Athearn dummy units used a different truck that had no internal gears and was very much like a freight car truck in the way that the wheels snapped in place - it had to be replaced if you wanted to power the unit.
Here's a modified truck from an Athearn switcher. The worm cover and the worm and bearings are removed in this photo, but you can see the small cover still in place on the inboard end of the truck, near the frame of the loco. The bottom cover is longer, but similar.
Here's and old LifeLike (Proto No-Thousand)

that I bought via mail order many years ago. The handrails were heavy plastic castings that were part of the walkway, there were mounting slots in the sides of the hood and cab, and the motor was mounted on one truck and was a very poor runner. I removed the motor and all internal gears, then redetailed the body shell. The two that I have are the only dummies on the layout, and needless to say, they don't pull very well.

On the rare occasions they're used, they run with a pair of Atlas RS-11s.
Wayne