Hi Shamrock,
Regarding steamplant-
Shamrock has a locomotive style boiler- these are not made for marine modelling so you would have to get it made specially- this alone could be round £1500-£2000.
The Engine- Shamrock's engine was a design that Roger adapted while in the navy, he used the cylinder block off a Duplex steam pump- and your right no-one does make an engine like it in model circles.
I have designed a launch myself that is 8ft in length based on shamrock's lines- I built the bow section and the cabin so far, I don't know if it'll get finished though, a bit impractical and i've learned a hell of a lot about steam since then- I was 16 when I drew the plans having spent an evening on Shamrock with my first training engineer on Gondola.
The woods I used were;
Hull- Oak keel, steam-bent oak ribs, oak knees and seat frames. Cedar strip planking brass nailed to the ribs. Teak/mahogany decks.
Cabin- Teak window frames, mahogany roof timbers (used the back of a guitar soundbox to make a really interesting curved roof) and pine panelling inside.
Regarding the steam plant I was going to use a 2 1/2" Gauge locomotive boiler design and shorten the barrel and make it side firing (this is how Roger made his boiler for Shamrock, it is the original boiler from River Irt, one of the loco's on the Ravenglass And Eskdale railway)- at this size you could gas fire for ease of use- or if you were clever enough you could even coal fire- no different to a model steam loco's grate size- just put a pressure controlled bypass valve on a permanent water feed pump and away you go!
Regarding the engine for the boiler- I would PM Bogstandard- his engineering is first class and has much experience with model steam- he is always busy but you may be able to come to some arrangement, failing that a Stuart Turner twin launch would be your first choice of engine- right size, right power, contemporary looks. You do know that originally Shamrock was fitted with a Sissons triple expansion steam engine- the rolls royce of small steam plants- i'm sure i've seen a 1:12 model of one of these, it wouldn't be cheap but imagine one of those in your launch!!

Regarding the engine size- see the thread in tugs and towing section on 'Evanos/Mont Caume'- I explained to this gentleman how to calculate power from any given model steam engine.
The power is slightly irrelevant though as Shamrock's lines are just so efficient- anything with a cylinder size of 14-18mm Sq. would be adequate.
Greg
Ps- about Fred Dibnah's series- he had engine problems with his tractor so Roger sorted them for him and then Fred went down to the boathouse the next day to have trip on Shamrock,
unfortunately fate being what it is, if he hadn't had engine trouble he would have had a trip on SY Gondola on the day Roger fettled the cylinders on the tractor- we were gutted as this would have been the best publicity for the boat it's had in years, hey ho!
