thats pricy for a non running ski. cut it in half. (and knowing hours on the crank is helpful too)
unless you've rebuilt engines before, your looking at an adventure, but honestly if you have then you would already know that $1000 is too much. I just passed on the same ski two days ago for $600, partially because I didn't need the project that bad, but mainly because the owner didn't want to take $500, i would have drove it home for 500.
top end's go bad for lots of reasons, and you have to keep in mind that you need to find out the reason during diagnostics, or you'll cook another engine. so you'd have to factor in.
top end, gaskets, labor, shipping of cores, bottom end tear down, inspect and or replace crank, more gaskets, pull carbs, inspect/rebuild, drain fuel tank, change oil, maybe new mats, buffing, non working gauges ? rusty cables ? pull pump, inspect, rebuild if necessary, etc... all the while trying to figure out what cooked the pistons in the first place, etc... like I said... an adventure.
you could just replace the whole engine for about 1K, and get a warranty.. not that much more than what your investment will be if you factor time and money for a home rebuild.
$500-600 is a good entry point, $1000 is not... and when you fix it all up, and invest all that time and money, the ski will be worth about $2,000 maybe a little more. you could buy a running one for less if you shop around, save yourself a lot of time and trouble.
the only upside to this is that you'll know that the engine is now "solid" vs buying a running ski and not knowing how long it will last.