I am pretty sure there have been a few built using the 11hp and worked well. I think the 6.5hp has been said to be a little under powered if you want to do deep water starts.
Remember the 11 hp weighs a fair bit more. If the 6.5 has a governer you might get more like 8 hp out of it by removing the plate. I was looking into building one a while ago and was told 8hp should be about all you need to pull a 200+ lb rider out.