Hello johnsonbar,
Just read this post, Dennis has half answered your question but MJ only supply some of the parts for a 5" gauge Merchant Navy. There are also well publicised discrepancies on the design which should be well researched before you start. A brilliant design if you want to haul 25 passengers, I admire the people who build beasts like this.
I had it in mind to draw up a 3 1/2" vesion of the unmodified(Spam Can) version. Even in this gauge it would be powerful loco, no doubt slipping even better than the full size. Rebuilt locos have also been constructed in this gauge but no published designs exist. As with my Q1 design, it's good to start with the unique wheel. LBSC designed 'Pamela' over 60 years ago and a wheel casting used to be available from Reeves. I think I bought the last two about 7 years ago. It's not very accurate, a much better product could be made by CNC milling.
As I live midway between the Mid Hants and Swanage railways I really should get on and design one, there must be at least ten of them in this area to go and peer at. I'll put it on the list of things to do when I retire (from the day job).
Cheers
Nick