Bob,
I think I'd have to agree with John. Plastic and steam aren't exactly the best of friends and I think there would have to be quite a bit of modification to your boat. Not impossible by any means, but certainly not simple or easy. The plastic superstructure wouldn't be my main consern, but removing the heat from the hull area would be a trick. Also, since I'm not familiar with the 'Lackawanna' kit, size mainly, is there room for a steam engine in it? [Not that familiar with steam engines either - lol.] Steam powered boats don't have to be huge, but they can't be too small due to the size of a steam engine. Have fun with it no matter what you end up doing!
- 'Doc

PS - Have a friend who does the steam train thing, engine is about 4 feet long (he rides it, no idea what gauge that would be), track about a foot between rails. He's crazier than I am! ... Wish I could do it.

PSS - Just found the 'Lackawanna'! Ought to be big enough, but still have doubts about the plastic hull.