MFA do a range of brushed electric motors that would be suitable. Take a look at the attached PDF file that gives all the info you should need.... I would suggest the 919D501 at 6v-8.4 volts depending on how much RPM you need - about 150-180RPM is the normal unloaded speed for most paddlers, and this motor would give 145 RPM at 6v, rising to 200RPM at 8.4v.
Hope that helps?
Regards
Eddy