Good luck with your build. I hope you found some useful info on sails and rigging. I agree the class rules are complex but are designed to provide a level playing field between contestants.
For your purpose the main aim is having some fun with your grandsons. I agree with your sentiment and provided you follow the basics the result will be fine. if I may make a small contribution, I suggest you make the mast position variable (1/2" either side of normal) so that you can balance the sails so the yacht behaves in all winds.
Please let us know how you progress

Thanks Dave and tiggy.
I have spent this last month reading about the marblehead class rules, sail design, how sails work, laminer air flow over sails, luff folds etc.

Didn't understand a word of it.

I will finish the Genie as a radio controlled pond yacht for me and my grandsons to have some fun with and build it to the specifications of

IF IT LOOKS RIGHT IT WILL PROBABLY WORK OK.

Thanks Grandad

The Genie was published in Model Boats as a series of articles over the latter half of 1973/early 1974. if I remember correctly the articles covered both r/c and vane versions. Plans for the boat are still available

http://www.myhobbystore.co.uk/product/27870/mm1175-genie

HI grandad and welcome to the site.
How you complete the Genie depends on what you wish to do with the finished model. if you intend to compete the model in vintage events then you would need to comply with any racing rules re design/sails etc. As an ex flyer I agree that older proven designs still work well but will not be as fast as the more recent offerings but will handle just as well once you get some experience.
I am not familiar with the Genie so can only offer some general practical advice. Some early yachts used to use a mechanical vane steering mechanism and did not have radio control, modern models are fully radio controlled for both the sails and rudder. Unless you have a suitable pond with all round access a Vane steering model would be a bit of a non starter. I suggest you look at http://www.vmyg.org.uk/ for details.
If you can find a local model boat club this would be of great help in deciding how to proceed as they will have the knowledge and skill to get you on the water and enjoying sailing.
Hopefully another member will have owned a Genie and will be able to give more detailed help.
Good luck and please keep us posted on your progress 😀