You haven't mentioned the type of motor or its RPM.
The propulsor on the Trafalgar has a coarse pitch and lots of blade area, so you're going to be needing low RPM, and plenty of torque to swing it around, I would estimate somewhere between 2-3000RPM. Doubtful your motor meets that criteria without gear reduction. I would say the motor /gearbox combination you have in the Vanguard would be closer to what you need, although the propulsor is somewhat different.
I think the Sheerline boats use a 12 volt Speed 400 (385) type motor geared down about 6:1, Chris could confirm or deny that. Having watched a few Sheerline Trafalgars in action, they always look a little slow to me- I would prefer a bit more performance, as these boats are no slouch in real life. Officially their submerged speed is listed as 32knots, but the real figures are always classified.
The Trafalgars I have looked at do not have any pre-swirl stators moulded into the shroud like the Vanguard has. The exception to this is Ramesh's version, Steve Reichmuth built a new shroud with stators on. This provides the counter torque that prevents the boat leaning/twisting over when under way. So if you go for higher power, perhaps this could become an issue.
If running lower voltage motors, will your water pump make good pressure on lower volts? I've found geared pumps tend to drop off a bit when you run them on lower voltage, so you might want to test that out for yourself. Also the the tank will take longer to fill/empty.