Hello
I had the original alu bracket on my H733. It is a bit heavy with twin Mercury 150 four strokes and I am building a new full flotation hull extension/ bracket. A naval architect calculated than the extension should be 23 cm longer and 30 cm wider than the original alu bracket.
There is a very long thread on the North America section on the Willard 730 restoration and there is a link to the USA bracket manufacturer.