That's a nice boat, I'd just replace the stern drives and be done with it, unless you do a really good job with the outboards resale would be hurt.
He designed his own based on a livingston style hull, not really the best choice IMHO. It was a crazy amount of work to design and build from...
Here's my pick, I sailed quite a bit on this one. http://simpsonmarinedesign.com/boats/64ft-pilothouse-ketch/
It's a copy of a 21 foot Skater. They go so well because they are basically planning from a standstill and the tunnel creates lift plus low wetted...