Sure it is the motor? Could be the jet pump. Need to disconnect before removing engine anyway so do it first.
Squirt a bunch of break-free type stuff into the spark plug holes and let it sit for a day or so then see if it loosens up by pipe wrench on the pto or a dowell into the spark plug holes and gently tapping with a hammer. Will still probably need to tear it down, but it will make you feel better if it actually turns over.
Welcome to the best SeaDoo forum, a lot of good information and helpful experts will be found here.
Yea, could be a big project, but fun.
Better off to start a new thread on this in the 2-stroke area so it will be seen by a larger population.
And pictures, we love pictures.
Rod