I'm not sure what the motor is, it's a 540 type and low current drain. I think a Monoperm would be too small and a Decaperm too big for this boat. I would recommend a low drain 540 type such as the Electronize 545-12 although that looks a bit pricey to me. Somebody may be able to suggest a cheaper alternative. As you can see, I have geared the motor down using pulleys and O Rings as driving bands. This allows the motor to run more efficiently and also enabled me to tuck it away to give more room for the other gear. If you do this don't stretch the O rings as this will bind the drive and current consumption will shoot up. You are correct that the battery is a 6v 7amp hour - lasts for ages with this setup. The only thing I would warn you about is that the boat steers fine when going ahead but there is virtually no steering astern, it just pulls to one side even with the rudder hard over the other way. I've tried using different props but it seems to make no difference so I guess it's down to the hull shape.
You don't say if you've bought a completed model or the kit. If it is the kit then I could send you a copy of the review I did for the now defunct Radio Control Boat Modeller magazine in 1992 which might help you with the construction. I found it to be a very enjoyable kit to build. Everything went together well and it resulted in a really attractive model which looks good on the pond and ashore.
Colin