Very nice score and the price was right. Looks like it is cleaning up very well w/o a lot of dings and dents.
At this point you're pretty much open to redesign it the way you'd like. Oak ply would look nice but I don't know how well it would hold up in a marine environment. All oak panels I have seen are interior grade and the glues are not water proof. Also stay away from any pressure treated stuff as it reacts with the aluminum.
Whatever wood you use will need to be sealed also. I was given a recipe for a sealer that I've found works very well and is inexpensive. It's 1 part boiled linseed oil, 2 parts mineral spirits and 1 part spar urethane. It's put on very heavy, until it starts to pool then allowed to dry for 48hrs., longer is better. then apply 2-3 coats of straight spar.
We have a Rich Line that is very similar to yours, in fact I 1st thought it to be a Crestline or a Lone Star. We found a 1960 Johnson 18hp on CL for next to nothing as it wasn't running. I pulled the magneto plate and cleaned up everything, reset the points and then cleaned the carb. Took a few pulls but it fired up and is running very nicely. Still needs a seal kit in the lower unit but that will be done over the winter.
That 18hp pushes this 14' very well even with 3 people in it. Total investment in this engine is about $50.00