For summertime, my hatch boards are replaced with a couple doors with removable Lexan windows and screens. The rest of the boat is fairly air-tight, so I guess a quality air-conditioner, one big enough for a van, would do a good job.
I have a generator at home, but it's an 8,000-watt monster that weighs a ton and is very noisy. The boat's engine is a Universal Atomic-4 gasoline that is really quiet, and I can open two back hatches that would vent the entire engine compartment so the engine heat wouldn't be trapped and heat the cabin. I think the installation would, however, be cost prohibitive unless you did it yourself. I would install it if I were 20 years younger, but I'm too old and fat to crawl into the engine compartment.