You can get a plan for the Mount Washington paddle steamer from John Fryant's Paddlewheels and Props website at http://www.steamboats.org/modelplans/index.html. Mt Washington was an excursion paddlewheel side-wheeler steamer on Lake Wiinnepesauke, New Hampshire, and was powered by a walking beam engine.
This plan is the one by John Breynaert, that is the basis for the Dumas kit, and includes a drawing of the complete walking beam engine. You can get to the listing for the Mt Washington Plans at this link: http://www.steamboats.org/modelplans/p4-washington.html
Pete G
Better yet!!! I just re-checked John's Paddlewheel Steamer Drawings listing, and he also offers a drawing for the Mary Powell, which should really be a great help. Three sheets drawn to 1/96th scale show outboard profile, hull lines, deck plans and engine details. Brief history and complete color scheme included. Model size: 37'' long by 8'' beam. http://www.steamboats.org/modelplans/p4-mary-powell.html
Should you have any questions about the fidelity and quality associated with John Fryant's own work and his offerings, this page may help:
http://steamboats.com/museum/fryant.html