Russell
There are basically two commercial units; Graupner and Robbe. The Graupner unit is based on a Speed 400 motor and has a tube diameter only of around 14mm. Robbe make two units, both of which would be more suitable. I'd tell you more but you can Google "Robbe" as easily as I can, and Westbourne Models will give you the right advice anyway. If you want to DIY then delve deeper into this Forum but beware! Here be dragons!
As regards controlling them, again you have two options. One is to use two microswitches on top of a servo. There are numerous clever people on this Forum who can give you the wiring diagram - it may even be here already. This will give you Full Port or Full Starboard only. The other option is to use an electronic speed controller, operated from the LH stick of your 4 - 6 channel transmitter. This will allow you to control both the speed and direction of the flow, and won't cost much more than a servo plus two suitable microswitches.
Best to consult the kit manufacturers and see what they used - they've built at least one more Spindrift than you have, after all.
Suit yourself.
FLJ