The outboard drive is shown in the pics. A small diameter high quality motor with a solid cylindrical body is needed. I found surplus Faulhaber ball bearing motors, a bit less than an inch in diameter, for about $7. Don't use an expensive motor. Eventually these drives will likely leak, and they are pretty much designed to be throw-away. With a $7 motor, a $5 seal, and some other bits, the cost is maybe $15 for each drive assembly.
The seal used is from Mike's SubWorks:
BHS - 1 Linkage Seals
Designed specifically for submarines, the BHS - 1 Seal can be used in any application that requires sealing a 1/8" diameter link. Size: .375" x .600" Sold in packages of 4.
I used an ABS pipe cap on the stub end of the motor, attached and sealed with JB Quick, the fast setting version of JB Weld. I think it's by far the best stuff for this application - waterproof, strong and stays flexible. Easy to work and control - sets very fast. I had to open up the inside dia of the cap a bit, and I turned the ouside dia down on a lathe (can be sanded too) to make it "sleeker". Not necessary, but looks better.
For the shaft end of the motor, drill an exactly centered hole in another plastic end-cap for the brass shaft seal. If the motor shaft is too short or doesn't match the seal diameter (mine didn't), then build up/extend the shaft with various diameter, short, telescoping pieces of brass tubing. The first units I built, I extended the shafts with a piece of cut-off M4 SS metric bolt so the Graupner M4 threaded prop could be spun on. On latest units, I just get a 1/4 shaft thru the seal, put some JBWeld on the end and stick on the prop, carefully centered. That's the easiest way and results in the shortest overall shaft length, desirable to minimize wobble stress on the seal. Probaly better to be able to change the prop - your preference.
Props that work are Graupner 50 or 60MM 3 or 4 blade. Right/left prop rotation direction doesn't seem a big factor. Prop RPM isn't that high, and with the rudder separating the props, co- or counter-rotation seems unimportant.
The motor leads are brought out of the end cap along grooves filed inside the end-cap. The JB Weld used to attach the endcap seals the wires from leakage.
The two sets of motor leads are joined and attached to a small Dean's plug the plugs into the hull, just adjacent to the top of the rudder.
Stress on the rudder seems minimal because the load is balanced. Turning performance, even backward, is very effective with the drives turning with the rudder.
I used an old plastic tray to make the mounting bracket. Anything will work, and since the unit doesn't show, looks aren't critical. Best to keep it EZon/off.
Here are some earlier posts with additional info showing original design, etc.