I agree with the above posters... I bought a 1966 20hp Johnson for $200, though it is a bit beat up. I'd say $400-$600 depending on its condition, but you'll find better deals on newer blocks at $875.
I've got a 14'4" 1960's Lonestar that I'm converting into a duck/buck/crappie chaser. I'm nearing the decking stage and I want to get the weight distribution correct the first time.
The 20hp Johnson and gas can go in the back.
The switch plate goes in the back.
The livewell can be up front or...