Depends on the class you choose to run and the Organization you choose to run with...
In the USLMRA stock class requires a full, original equipment deck (blades removed)
All other class's (besides F/X) require "Deck Halves" to be mounted on the mower...these can be cut down pieces from another mower mounted to yours...
In F/X (Factory Experimental) you can run a "deck like" foot rest as long as the result "looks" like a mower...