Bob,
Fibreglassing? Beats me, but it would certainly be 'better' than just plain plastic. The conducted heat from the firebox and the radiated heat from the boiler would be the 'biggies' I'd think. The only steam engine I've any experience with at all is a very simple one, firebox sits on a small block of wood, then the boiler on top of that. Tried making a plastic base (bad idea!) which didn't exactly work out. Didn't melt, but retaining pins got very loose, started to smell a bit. I'm not totally stupid so removed the plastic, put it back on the wooden block. The engine sits out in the open so didn't worry about the 'contained' heat in the hull.
Constructing a 'boiler room' may be an option. Contains the heat away from the hull, give it a vent through the stack (also an air intake). Depending on the engine, I'd also think about a bilge pump. I've found that there's a lot of steam/water blow-by from my oscilating engine (more common knowledge that I 'learned' the hard way).
There are some real 'steam boat' nuts around so finding them would be much smarted than listening to me! Build the thing, make notes, write it all up, make a fortune in the R/C modeling world! ...
- 'Doc

PS - The overhead power lines are a very nice touch!