IDEALLY, if you had a compression tester, you could check to see if both cylinders had compression within 10 percent (or less)of each other, then you could assume the pistons and cylinders are in pretty good shape. If it ran pretty well during the time you got to ride it last winter, they are probably good to go. IDEALLY, you could install 2 new pistons while the motor is out, but that's gonna add $100 bucks to the project and another 3 or 4 hours time.
Removing the motor for the 1st time, allow a couple hours. Then, the battle to remove the flywheel begins.
If your pistons and cylinders are OK, and you are just replacing the crank seals, you can lay the motor on a workbench with the cylinder heads down. Then you can just remove the the bolts from the bottom of the motor that hold the crankcase halves together. Then you can get to the crankshaft with the seals on each end. There is a pressed on gear on one end of the crank that you gotta remove to replace the seal on that end.
You have to clean everything up well, particularly the two mating halves of the crankcase, which will get sealed back together with some Yamabond or ThreeBond 1184 sealant....don't use regular silicone. I'm sure if you entered "crankcase sealant" in the search part of this site, you will find some other good sealants.....I can't remember their names. I have always used the ThreeBond stuff, with good luck. It used to be called ThreeBond 1194, but the new, environmentally friendlier formula
is now called 1184.
All told, it would be a pretty good 8 hour day for someone that is doing it the first time. And that's if you don't have to fight too long removing the drive clutch, the flywheel, or the crankshaft gear.
But, break it up into segments. Remove the motor one day. Tear it apart to change the seals another, and install it whenever you get a couple hours.....
Take your time...you will probably have questions as you work through this.