John's Steam Tramp has an efficient ceramic burner, the Midwest single cylinder oscillator motor, and no ventilation other than the smokestack. The efficient burner means that there is little waste heat to vent through the smokestack; you can hold your hand over the smokestack of both the Tramp and the AJ.Goddard without getting burned (both ceramic burners are homemade, btw). The Goddard's boiler is on deck, so needs no cooling to protect superstructure. The Tramps engineroom/boiler room is encased by ceramic blanket and aluminum flashing, which you can see in the build photos.
Hookpilot your setup will be much more elaborate, and generate much more heat, of course. I just mention the Tramp and Goddard so you can get ideas on burners that don't shed much waste heat, and some insulation tips. Ceramic blanket insulation, as used by the 2 steamers, would help protect the superstructure sides and ceiling from heat, if you have the room to install it. The fan advice you've received, along with opening some windows and doors and adding some insulation, will, I think, provide enough cooling.
You might consider a bench test of the steam/boiler system w/o the superstructure. Hold some scrap plastic/wood/metal bits near the hot areas and see if they are going to be damaged, before you set the superstructure with it's great details, over the boiler room. If you find heat problems, a "tea cozy" made of ceramic blanket to cover the engines and boiler may help.
Goddard (ceramic insulation report#21)