I would suggest a Mabuchi 555 motor running on 12v
http://www.componentshop.co.uk/mabuchi-555-dc-motor-mounting-bracket.htmlComponent Shop can also supply ACTion electronic speed controllers; a P79 would be just the ticket with this motor.
Model Boat Bits can supply suitable prop shafts and tubes, couplings, propellers, rudders and tillers
http://www.modelboatbits.com/There should be some guidance on the plan (assuming you got one with the hull) as to what size prop and rudder you require. If not then a 40mm 3-blade plastic prop would be a good place to start.; you don't want the thing to plane across the pond!
I think that Scoonies have quite a range of fittings of their own make but a quick Google will reveal others. Beware of ordering any Graupner or Robbe fittings, however. The former are now unobtainable and the latter are being slowly taken over by Krick and may be on long delivery dates. There are some UK mail order suppliers who will cheerfully take your order and your money for goods which they don't have in stock. None of the suppliers I've mentioned have ever given me anything less than excellent and honest service.
Dave M