If it's Medway Bridge then those aren't 400hp they'll be 365 at most. As I remember when I looked it over years ago it had a bit of work done on one engine but not a lot.
The owner had been shafted by a yard and had bills as long as your arm but not for anything of value in comparison to what he had paid them.
So engines will be throw away really for what you want to do.
If you're really serious about Marathon you could have my XPS and re engine that and it would be a more capable proposition with money back on sale of engines / drives. Same designer, bit more length ooer missus