Did you check with these guys?http://www.texasoutboardrebuild.com/index.html
They are down past Cedar Creek; however, I think on their website the will ship a block and you install it.
These guys are the gurus, but it won't be cheap.http://www.hydrotecmarine.com
I haven't pulled a powerhead yet, but I would guess that you are going to have to strip and pull the existing head to take it to a machine shop to get the cylinder sleeved if it is really torn up.
The real question though is what caused the #6 failure. If that's not fixed, then the same will happen with a new head. Maybe Gary will chime in.