Congratulations on deciding to keep that boat! It's a rare and very special model that few people understand or fully appreciate it's capabilities! Forum member Big Fluke is a naval architect in Hawaii that has a Seafari 25 and says once you get CG location right, it's one of the most seaworthy small boat hulls he's ever tested!
Carl Moesly designed the Seafari 25 to have a ballast tank up forward, similar to what he used in his race boats.
http://www.moeslyseacraft.com/25-seafari.html However he sold the company to Potter before ever building a 25, and Potter apparently never understood the ballast tank so left it out. Potter has been quoted as saying that the 25 Seafari is the one boat he could never get to "handle right"! Connor Davis is the resident authority on this hull, with lots of experience running his boat in some big seas on several Bahamas trips; he says it runs and handles much better with weight up front.
Although I have a lot of experience with a bracket, having adding one to my boat after running it for 31 years with an OB on transom, I believe going to a bracketed outboard on the 25, ESPECIALLY one with no flotation, is a big mistake. The last thing that boat needs is a rearward shift in CG; if anything it needs to shift forward, so I'd consider installing a big fresh water tank up front. Dave (Blue Heron) and Strick, who bought their 25's with blown engines, decided to stay with an I/O to avoid shifting the CG and I suspect Connor would recommend the same. Although a big outboard, especially an E-TEC, will weigh much less than an I/O, most of the I/O's weight is forward of the transom, so an outboard on a bracket will shift the CG aft in the wrong direction. Carl told me that the CG on the 140 I/O Seafari 20 is further forward than it is on the OB model. Forum member Tiny who once owned both an I/O and OB Seafari 20 said that the I/O model does indeed plane easier at lower speed than the OB model and he felt that it rode a bit softer. I'm sure a comparison between an I/O and OB Seafari 25 would be similar.
I would suggest that a single I/O with a duoprop type outdrive, with a modern EFI V-8 like Blue Heron and Island Trader bought, would be the optimum configuration for that boat, and it would be lighter and thus perform better than the twin I/O's than it originally had.