Coles has the Challenger
There are plans on most of the Yahoo engine groups for the Amsbury and Whittle V8.
What size boat are you building? The Challenger is a 1/4 scale and would be a bit heavy.
I am currently making plans for a 5/8 bore and stroke V8 that is about 1/5 scale. The block is just short of 5 inches long. This way i can build it on my small mill and lathe. I will make the plans available but it wont be for a while.