Based on the link provided by @highprobability (I haven’t bought it, just looked at the pic) it looks like there’s enough space for a lightweight, compact V8 of some description. The first and most obvious candidate would be an aluminum GM LS, which tick all the boxes; cheap (if you find one in a junk yard or in a donor car), compact, powerful, fairly fuel efficient, lightweight (in the aluminum block variants) and with plenty of parts found anywhere. They also sound pretty good, and the correct header/exhaust combo could probably get you pretty close to an old school flathead sound without the other compromises.
Another V8 option would be the Toyota/Lexus 1UZ or 3UZ, or the replacement for that engine, the UR family. These are DOHC rather than OHV, so they rev a fair bit more but they still tick most of the other boxes as the LS while being possibly a more interesting choice (I’m a gearhead, and LS swaps have become almost passe online).
Or you could go with something that certainly wouldn’t sound traditional but might be a better performer; a 3.0-3.5L V6, namely the Toyota GR family, the Honda J series, the GM High Feature family or the Ford Cyclone. All of these are ubiquitous (they’re used in virtually all of the vehicles made by their respective manufacturers), efficient, compact and relatively powerful, and proven to be successful in sportier applications.
I’m not much of a boat guy, so I don’t really know exactly how a front mid mounted engine might affect handling, but my understanding is that too much weight up front can lead to bow steering. My gut is telling me that a compact V6 mounted behind the driver, where the gas tank is shown in the illustration, might be the better handling choice, with the drive sent forward and a V-drive used to connect to the prop shaft.