Assuming you had an original Volvo 90-110 HP Diesel, and you actually want to replace vs. rebuild, your engine selection is limited by RPM range and engine room height restrictions. If Insurance companies are involved, the typically go with later model Volvos. If you’re looking for a used replacement, the Yanmar 4JH (85-110 HP) and 4LH (140-235 HP) diesels are economical, fit well, and give very similar torque & HP curves. The 4JH series are plentiful and take-outs can usually be found in FL. The 4LH series are harder to find but the 140 HP version (4LH-HTE) is also an appropriate swap. There are at least two of these LH repowered Manatees here in Sarasota for you to look at.
The old faithful Kubota 85 would be another candidate, but It’s a bit too tall I think. John Deere 4045 is a dream but is also too tall. The first 12 or so Manatees had 50 HP Perkins, which took the boat to hull speed but there was little power left for additional accessories, so I wouldn’t recommend it.
Try to keep in mind that the only thing that a manatee will do with more than 110 hp is just dig a bigger hole in the water. The top speed of my boat with a clean prop & bottom is 9 knots and that is with the swim platform totally submerged. Don’t waste your time & money on more horsepower.
You mentioned Genset replacement too. A good used Northern Lights 5 KW can be 5K or more. You may also want to consider moving the unit out from beneath that terrible space below the galley and into a new hatch in the veranda. It changes everything.