My first consideration would be that,
I do not recall there being a reverse on those jet ski.
If you don't care about scale then I would consider looking for a
"Z-drive" from graupner or robbe. it is like a small trolling motor
designed for r/c tugs.
If you have a look at this LST build up, you will see how we typically run
propeller shafts.
http://groups.msn.com/ModelersAndHobbyForum/japaneset103lstbuildup.msnw?Page=2
I see a lot of people have difficulty with propeller shafts, stuffing tubes, and hull mounting and sealing. I may have to do a quick tutorial on this.
I will have to get some good image of a typical shaft installation the next time we place some.
1/144 should have little difficulty running off one or two shafts. The stuffing tubes can be made up of KS brass rod and tubing.
Just start with at 1/8 inch rod, and get the next three sizes of tubing that fit over the rod and each other.
A short 5/32nd inch piece of the smallest diameter tube, will be the bearing. A 1/4" piece of the next size larger, the bushing, . The Stuffing tube being the largest is cut to a length to allow the tube to reach from the propeller thru the hull, and then to a height to meet the motor shaft. The bearing and bushing are soldered one inside the other, and then both are soldered into the end of the stuffing tube. One set for each end.
I then cut a long hole in the bottom of the hull to get the stuffing tube at the correct angle. I tape an block the ends into place, then I put tape along side, and over the stuffing tube so that the tape conforms to the outside of the hull. Once the tape completely seals the hole around the stuffing tube, I pour five minute epoxy into the space around the stuffing tube on the inside of the hull and let it harden.
When the tape is removed from the hull, the epoxy will have cured conforming to the hull bottom. Then just a little sanding is required to clean up the edges. Once the hull is painted no one will see how big a hole you cut to put the shaft through.
edit:
Once the shafts are in place, you can try an unlimited number of motor, battery, and propeller configurations for speed and running time.
We typically run 12- 14 volt Pittman motors. Our running times for our 1/72nd scale ships on a 12 volt 3 ahr battery are about one to two hours of continuous running. On a typical sailing day, where you are chatting, and running the ship intermitently, we never need more than one batteries for the day.
Aimee