Hi. I know of a Lowell company that built aluminum boats in the early 60's in Spokane, WA.
What's the beam (width) of this boat, and what is the hull shape? It's probably before the time boats had to have plates telling you what the max hp was.
If it's the typical semi-V hull, around 60" wide or so, it's probably good for about 25 hp. If it's a rounded hull, like the old Alumacrafts, it's probably right for about 15 hp.
If it's narrower than 60", then the hp goes down.
I once had an older Lund 14' boat that was actually rated for 50hp, but it had a beam of 72" and was very deep, with a full 20" transom.
One other question? Is the transom cut away for the 15" engine height, or is the transom just straight across?