I'd have thought a marine engine would be awkward - aside from finding a gearbox to fit it (assuming the existing engine 'box is designed for a two speed propeller) I'd expect parts to be very expensive.
Just based on my experience servicing my boat engine - even the smallest parts cost a fair bit.
Not to mention if it's a used boat, you'd have possible salt water corrosion (especially on the water cooling as most boat engines obviously use water sucked up from around them)...
That said, the engines pictured do look very shiny and I bet they'd sound nice!
EDIT: Actually, just looked at the cost:
http://www.theboatcentre.co.nz/price_specs.html <- I know that's in Dollars, but yikes!! The 300 linked in the second link by the OP is just over NZ $45,000 :O