A few years ago, a friend of mine and I did an online search of the firms still making and selling everything from plans to hardware for steam launches. It turns out that building something like the Squib (about a 30 footer) is no problem. One outfit sells the castings for a copy of a triple expansion torpedo boat engine from the 1870's. We also looked at the Davids. It turns out that the Davids have several problems. They tend to tuck under in a swell, wouldn't steer particularly well and anything over about 50% in a replica wouldn't transport on a flatbed. There is the possiblity of building a David in three sections. Bow, stern and center with bulkheads. Cosmetically, you could hide this pretty well. When hauling, you would have the center section level in a cradle and the bow and stern would be vertical at the ends of the trailer. You need a quick release for the drive shaft. Don't try to build or operate one of these without the outward canted railing around the cuddy. They were designed to take water flowing up the side of the hull toward the open area and turn it back without flooding the cockpit. You could make an interesting approximation of the spar and torpedo using aluminum tube and one of those aluminium mini-kegs. An old friend from engineering days told me that they have technology used in movies that would allow a sort of "mortar" they would blow out through vents from the keg (it wouldn't explode) which would probably make a very satisfactory simulation of an explosion in water. If you went with a steam engine, go propane fired with some real coal and a clinker box for realism. OH, don't forget the hinged cap for the smokestack, the original David didn't have one and had her fires extinguised when water from the explosion on the New Ironsides came down the stack. There is a company in Oregon that makes and sells plans or complete hulls that would be a fair fine-nosed launch approximating the Navy torpedo-boat classes (domestic).