Thank you both very much for your replies, not least for the recommendation of Dodgy Geezer's excellent website, which I have consulted extensively during the build process so far.

I am minded to agree that careful programming of the ESC is likely to prove the most effective means of guarding against untoward overpowering.

I shall endeavour to upload photos/videos of the boat in due course. if something goes spectacularly wrong as a result of the motor setup, let's hope that it is at least caught on camera!

HI colecanth
I think haverlock must have missed the bit about the gearbox and was referring to direct drive.
Your motor with the gearbox will not overpower the model and the props will be suitable.
The original may well have used a different motor and prop but as you already have a suitable set up you can and should use what you have. if the power is a bit high you can reduce the battery voltage, but I am assuming you will be using an ESC so will have full control over the speed.
I tried the web site suggested but couldn't connect, http://eezebilt.tk/range.html shows the range of kits and may help with your build.
Dave 😀

Woooooooooooah sorry but both of your motors will overpower your boat by a BIG margin !

When the kit was new the motor fitted ( according to Keil Kraft would have been a 380 max and a working voltage of 4.5 volts.

The kit prop was a 2 bladed and if memory serves about 15mm ( very crude ).

http://eezebilt.co.vu
Check that website out ( if it comes back up) it has a lot of data about the original kits and how to make props etc.

HI coelacanth
Welcome to the site.
Your model is a classic from an earlier age and at 14" will not need a great deal of power to sail at scale speed.
As you have already incorporated the gearbox into your model I suggest you stick with the original motor and choose the prop that gives the best performance and sail time.
If you were to use the brushed motor I suspect your gearbox would be running at many times its designed speed and I doubt if the bearings (if any) would last for long. Worst case it would just seize up/explode and stall the motor which would be destroyed, probably take out the ESC and possibly cause the battery to overheat with the risk of fire.
If you were to use a brushless without the gearbox the smaller prop should be OK but the speed will be over scale, so I share your misgivings.
Weight will be your main concern but a 7.2v NiMh in the right dimensions should suit.
Using brushless and LiPo will result in a very light model. I am assuming you have experience of LiPo and their safe usage.
Good luck and please keep us posted with progress.
Dave 😀