There are several repairs that you can make without disassembling the e-unit:
An e-unit that sticks in the up position may have notches worn into the crotches between the teeth, which catch the pawl and keep it from dropping. It is possible carefully to even out the side of each tooth that the pawl slides on with a small wood chisel.
Another cause of sticking seems to be roughness at the point where the heel of the pawl hits the bottom of the plate under the solenoid (n.b. spelling). A bit of tape here may help, at least temporarily.
A finger that doesn't touch the drum or touches it too lightly can be tightened by bending it right where it leaves the wiring board. Make an L-shaped hook from a paper clip that you can pull on to hold the finger tightly against the edge of the board (no bending yet). Then carefully bend the finger back toward the drum by pressing it with a small screwdriver at a point just barely past the paper-clip hook.
The fingers end in a C-shaped curve where they touch the drum. Eventually, this will wear until the outer half of the C will fall off. You can get another half-century of life by flattening the tip of the finger slightly, so that the original 90-degree angle between the finger and the start of the C becomes about 45 degrees. Then the middle of the remaining half of the C takes over the job of contacting the drum. You will probably also have to use the preceding trick to tighten up the finger.
The 671-50 and 726-51 e-units are the ones with springs, for horizontal mounting. You can easily convert a vertical unit for horizontal use by putting a suitable spring into the central tube. I did this for my MTH Big Boy, until I was able to find the proper unit. There is also a vertical unit without the shutoff lever, the 773-300.
American Flyer e-units can be used in Lionel-type locomotives. As you might expect, they are smaller and mount horizontally. They get by with 4 rather than 6 fingers; and the shutoff is mechanical rather than electrical.
