It's deliberately not difficult -- per FAST - Autocross :
We have eight classes, loosely categorized by displacement, modifications, and tires. Clarifications are listed below the classes.

• A - Modified race tire (all)
• B - Production race tire 3 liters and greater
• C - Production race tire less than 3 liters
• D - Modified street tire 3 liters and greater
• E - Modified street tire less than 3 liters
• F - Production street tire 3 liters and greater
• G - Production street tire less than 3 liters
• R - Open or closed wheel race cars (all)

Modified cars are production-based cars with any of the following modifications:

• Any internal engine modification (crank, cams, rods, pistons)
• Any external engine modification that alters fuel delivery (alternate carb, altered fuel injection, fuel-air ratio adjusters)
• Any non-factory engine replacement
• Welded chassis reinforcement, including welded roll bars/cages
• Alternate or altered springs
• Altered suspension geometry (relocated pickup points, alternate suspension parts to increase factory alignment adjustment range such as camber kits, camber plates, or slotted strut mounts)
• Alternate main body panels (doors, hood, trunk lid, fenders, roof panel, windows)
• Removal of factory interior parts (seats, interior panels)

Other classing notes:

• Any and all modifications not listed are allowed in the production classes.
• Street tires are DOT approved tires with treadwear rating of 140 or above.
• Forced induction (turbo or supercharger) bumps engine displacement by a factor of 1.5. (e.g. a 2.2 liter turbocharged engine would be classed as 3.1 liter)
• Any non-production based vehicle (such as kit cars or tube-framed "look-alikes") will be classed as a "Race Car" unless deemed an appropriate fit for a lesser class.

If you need to change your class, just go back to the entry list on MAE and edit your entry.