if you can't find a tag inside the boat(my 1972 didn't have one either) i've found from research that boats 12ft and under that age seem to have a 3-5hp limit. also remember that old 2 stroke motor were much lighter than new 4 strokes.
i bought a 72 johnson 5hp 2 stroke that weighs about 30lbs.
a newer mercury 5hp weighs closer to 60lbs. that's twice the weight and 4 times the price.