Ok, let's start with the fact that NONE of the afore mentioned RR's have ROOM for anything much bigger than a forney. WW&F will most likely be the first one to reach that point.
Seccond, by "joint venture" I meant a pooling of resources, much like having Boothbay build the boiler for #9. They can easily build one for #7.
If SR&RL wanted to get in on the act, they could likely shoot for a copy of SR&RL#10, or annother similar engine.
(Personally, I'd love to see a #24, but again, where would ANY group put it?)
But as for casting common parts etc, I think some cooperation can and should happen.
But, since you're talking size here, is the British loco you reference anything close in wheel, boiler or cylinder diamiter to anything that ran in Maine? As was mentioned previously, even if you did go to the Brits you'd need a series of "one-off's."
Personally, I'm all for keeping it as American as possible. There's enough good hard US $$ going overseas these days as it is. Let someone HERE benefit AND build up the skills and tech necessary for such projects.