I believe you’ll fall into the Street Touring Class rules which for an S2000 puts you in STR. The rulebook for ST states:
The driver and front passenger seats may be replaced with the following
restrictions. The seating surface must be fully upholstered. The top
of the seat, or an attached headrest, may not be below the center of the
driver’s head. The seat, including mounting hardware, must weigh at least
25 pounds and must be attached using the OE body mounting holes/studs.
Additional mounting points may be added