Roller bearings......
I'm not so sure the motor will need a new crank if the roller bearings seize from water. I'm not saying that it won't either. I believe this will be a case by case situation. Depending on whether the seized bearing got a flat spot in it while you were trying to break it loose.
I have a 650 that was locked up tighter than a drum. It had sank the year before and had set up with some water in it. I poured a good bit of lubricating solvent into the spark plug holes and intake and let it sit for a day or so. Then, rocking the engine a little each day, I was able to free it. It took about 3 days after I freed it, spinning it over manually, then by the starter, that I finally got it to fire. It smoked pretty bad but cleared up after a bit.
But, the bottom line is, after starting it, I haven't done anymore work to it but rebuild the carb and it's run ever since. That was 2 seasons ago.
So, in my experience with 2 stroke roller bearings, they can take a bit more punishment than we think they can.
I do wonder about the jugs freezing though. By design, the 951, 787, 717, 657, and 587.....their cooling system is a TLCS (total loss cooling system) and has a drain at the bottom of the engine for allowing all water in the cylinders water jackets to be expelled. There is probably a tablespoon of water or so that can be left but I've never had one freeze and crack. Also, my geographical location may play a part in this. Here, on the Gulf Coast, we don't get much below 32*F and then, it's only a few times a year.
Try and spin the motor by hand as suggested. I still think you may have a dead battery. Left outside, I think it'd freeze before the engine. Sulfuric acid in our batteries at a 33% concentration is susceptable to freezing, killing a battery. Although, specific gravity determines the freezing point and in batteries, varies on it's life cycle.
Let us all know if the engine spins over!..........:cheers: