I used Graupner 400's with 3-1 open gearboxes from Great Planes along with a Viper 25 esc. They work great. Some guys use the belt reduction with the 400 size motor and never regretted taking out the big 500 motors.
Engle has bayonette rings for several subs now on their website. This would be the way to go if you are building a new boat. They work great on the Layfaette.
The speed of the U-47 is fast with the 400 setup. I can run for several hours with the 2 6v 4amp gel cells I carry. They are in parallel so that you have 8 amps to run with. I also converted the boat to RCAB and use two bags to conttrol the diving. This is a great boat that is fun to run and looks great. There are several add on's available to enhance the looks of the kit.