That might not be quite so easy as "Mt. Washington" models are known for being rather top heavy. A hollowed out solid wooden hull would end up too heavy and with batteries, RC equipment and the superstructure built the way it is, you might well sink the boat below the sponson decks and she would very likely capsize in the turns. Model paddlers (and even real ones) can be very finnicky creatures!
You might consider leaving the model as it is and make a scratch built hull from its dimensions. If you REALLY want to do it properly, then there are plans available on Ebay for Mt. Washington, but I wouldn't recommend her as a first project. Hate to throw a damper on things, but try making a model with a low superstructure (i.e. low COG- Centre of gravity) first. Hulls are not difficult to make, so why not keep that nice model you purchased on Ebay as it was intended...as a pleasing display.
PJ
Victoria, BC Canada