Replacing seals would be a good idea, but not necessarily the crankshaft.
Try and wiggle the ends of the crankshaft, if it dont budge and the bearings are smooth then the bottom end crank bearings are good.
Then, test for play in the connecting rod. With the cyllinder off, pull up and down on the connecting rod, and also the piston on the end. If they have any slop up and down, its toast. A little side to side play on the connecting rod is okay.
Replacing the left side seal behind the flywheel will require you to remove the flywheel and stator plate, which will likely need an adjustment after you put them back. Be careful not to loose the flywheel key (should be a small pin in this case, usually doesnt fall out, but still be careful)