Fitting a 4HF1, a turbocharger and a water-to-air intercooler into a surplus American Ford E-350 DRW ambulance is not so hard as trying to make a 6-cylinder to find its way there, but it's far from being an easy job.

Power-to-weight ratio in small stern-drive boats is not so exciting. I don't understand why those 2-cylinder Yanmar and its Chinese rip-offs are still used for that matter, they should be restricted for the stationary usage they were initially intended for. Might be still OK for a kuliglig but on boats the dead weight is even more problematic. The cheapest is not everytime the best, there are many 4-cylinder engines that are not so heavy compared to the 2-cylinder stationary Yanmars, even though in certain cases displacing the double of them. I've seen the TDI engines from Audi being also available for marine purposes in other places, and due to the modular engine designs from the Volkswagen Group it woldn't be so hard to get them made in China too.