Yes you can use the stock front axle. But it isn't recommended because it will break.
Don't build to the super stock class. It doesn't exist anymore. I'm sure when they update the rule book this fall it will be gone.
Choose one of these classes if this is your first mower. IMOW, GP, AP, SP, or CP.
I'd recommend GP if you don't want to buy a billet flywheel. SP if one is in your budget.