The nice thing about the Johnson/Evinrude 60's and 70's is their weight, and a nice running 3 cylinder 2 stroke that fires every 120 degrees is about the smoothest running 2 stroke there is. I would try it as is before making any changes. Just make sure it's running right and turning up the rpms that will allow it to get into it's power band. Nobody cares about that much when just cruising, but for pulling, it can make a LOT of difference having a carefully selected prop installed.
Then, if not happy with the performance, as Tommy hints, you're going to need to be very careful with weight. The older toons don't have the big tubes the newer boats have, making them pretty weight sensitive with a tendency to sit way low in the back if over whelmed with a big motor