Re: Johnson Sailmaster 8hp
I have an 8 hp two stroke Nissan on my '79 Oday 23 and its plenty powerful. There is no difference in performance between the 8 and the 9.9 it replaced. I think that you can get away with less hp with a four stroke; 6 hp on a four stroke should be plenty. Whatever you buy, don't get anything other than a "long shaft" model. Some manufacturers make an extra long (25") too. I'd pay extra for that in a heartbeat.
If you are going to upgrade to a four stroke, make sure your mounting bracket is rated to take a four. Remember, its not just the extra weight of the four strokes that is at issue (in fact, the newer fours don't weigh that much more than a two of comp. hp), a four stroke generates more torque, especially at lower revs than a two, and the bracket has to be able to take it.