I answer your questions here as others may be interested in my reply, but you can also write to me direct at woodsdesigns@tiscali.co.uk
The standard Janus mast is 8m long so a Hobie mast would be about the right length. However you’d need to raise the gooseneck, or use the deep mast beam which, as I wrote yesterday, I recommend you do. Also you may find the fully battened jib a pain on a larger boat. Fortunately small monohull jibs are usually very cheap to buy (I have two for sale myself) or a jib from another beach cat is another option.
I’m not sure what glittering fittings Wharram has eliminated. Presumably he still uses mainsheet blocks etc? The Janus plans show how to make a wooden boom. You can use lanyards rather than bottle screws etc.
In any event the Janus uses very few parts not found on a beach cat. For example the beam bolts are lengths of 1in dia aluminium tubes pushed through plywood brackets. Very simple and cheap.
You could make a small cuddy, similar to the one on my website that I fitted to our Merlin last year. It would certainly improve comfort on board. However I’d keep it for sleeping in and put the galley in a hull, see the photos of Tucanu on the website, as that uses the space more efficiently. I’d also have a solid deck throughout, rather than have some trampoline area. Although the trampoline is lighter and makes it easier to trail the boat most people don’t like a “soft world”.
As Janus is built in wood it will float if holed. You would be very unlucky to get holes in both hulls, but even if you did it would still float. On my larger boats I make areas under the sail lockers and bunks into buoyancy compartments. These also make the boat double bottomed so if holed water shouldn’t enter the accommodation.
However the Janus hulls are so small that you need all the stowage space you can get. So it is a compromise. I reckon most damage is going to be in the bow sections so I make a crash bulkhead and the sail locker watertight. The space under the bunks can be watertight compartments or used as lockers. Personally I find the under bunk storage a pain to get to, so prefer hull side netting bags. On Tucanu only the aft end of the front bunk has stowage under.
I too need more photos. Your English is very good. The Janus study plan and partial plan set will give you a lot more information than that published on my website.
I hope this answers your immediate questions.