The 30 Sisu is a fine boat. They are in fact cored above & below the water line. It is important that a careful hull inspection is done around all thru hull fittings to make sure no water has entered. If all was done correctly then move on. A 260 Yanmar will push a 30 Susi easily to 15-16 knots on the cruise. Good power for the boat.
i have a 30ft Sisu as well without the yacht finish, she has a DD 453 that sips fuel and pushes it along at 10kts, they dont need alot of power due in part to there narrow beam to length ratio. also ones of this era are framed with wood, good fir mostly but you should look over for any de-lamination in frames, engine bed and deck.
maybe one of the builders on forum can tell you better about engine box question, i can say there is fair amount of room in the bilge for a good size diesel such as an older DD, with the newer, smaller, lighter engines such as the Yanmars i can see it could be flush with sole. if you IM me i can send you a name of a reputable surveyor out of Fairhaven, otherwise ask around and/or look for professional accreditation such as SAMS, ABYC designation, there is no license that i know of.
thats why you get a highly recommended surveryor or FG repair guy familiar with the boat and pay him to check it out. If all fittings were bedded correctly and there is no water intrusion to the balsa, cored hulls can make for an exceptionally stiff boat, with decent weight savings.