previous owner could of tried to work on it, and instead of taking the engine out before trying to repair it, could of just taken the thing apart while inside the hull, and the oil leaked out.
exactly how much is still inside the hull? can you take a picture and post it?
Congrats on a good ski. waveraiders and wave ventures are fun ski's.
This is what i would do, Clean out the inside of the hull very well as well as that sludge, take an oil and fuel sample out of the tanks (just to look for separation or water, can just pump it into a cup) . Next I would wait over night to see if the engine had a small leak anywhere, if not i would try turning the crank and see if the engine would turn over. ( at this time you can also see if while trying to crank or turnover the engine if it leaks anywhere).
Then id start the rebuild if all else fails.
Im interested in seeing what exactly is wrong with your ski, keep us updated, post pics when you can.
Thanks and Welcome to the Forum!