the merc powerheads are way different than the seadoo. merc has this huge rectangular hole just in front of the pump area that the gear train sits in. the merc jet motor crankshaft is vertical, like an outboard(go figure), and then it goes down below the water line in a housing and changes the power to horizontal. seadoo only has a small circular hole that the driveshaft goes through and the seadoo driveshaft is basically an extension of the crankshaft.
I've never seen it done and it would be a major project and not one I'd take on but anything is possible. The simplest way would be to take a 3'x5' engine and pump section from the rear of seadoo boat with the pump and motor and patch it in to your boat. They have amazing epoxies now for boat repair, see West Marine.
I'm currently doing a major boat repair on a 2005 twin engine 4tec 370hp Speedster that came off the trailer on the highway that went into a ditch, through a row of signs that punched 5 holes through the hull and flipped over. Insurance company totalled it out and I got it for $2400 but have about $4k into it so far. A sign post went right through the pump area behind the motor, through the water box, hit the motor and broke the valve cover, hit the timing chain(replaced that with the new timing chain recall kit, see 4tec thread).
Didn't mean to overtake your thread but just want you to know that anything is possible with enough work but I believe it also needs to make sense as far as money goes. You should take your best guess what you think it will be worth when it's done and how much it will cost before starting. This speedster I'm working on is a $15k boat so I have some room to work.
