I assume you already have this information, if not, it's straight off of AHRMA's website.
Of course no support class info listed though, that's likely up to the tracks discretion I would think?
Vintage Motocross Classes
Note: All classes are further divided into Novice, Intermediate and Expert rider ability levels.
Premier Lightweight: Certain pre-1965-era machines up to 250cc.
Premier 350: Certain 1960s-era four-stroke machines 300-350cc
Premier 500: Pre-1965-era machines up to 500cc
Premier Open Twins: Pre-1965 twin-cylinder machines 600cc and larger.
Classic 125: 125cc two-strokes and four-strokes up to 150cc, manufactured through December 1971 (and like-design).
Classic 250: Specific 250cc and smaller two-stroke machines introduced just after the Premier Lightweight era (i.e., four-speed Spanish motorcycles, twin-pipe CZ), as well as 350cc non-unit motorcycles.
Classic 500: Specific motorcycles from the late 1960s and early ’70s.
100cc Motocross: 88-100cc two-stroke and 88-120cc four-stroke motorcycles manufactured as 100s, up to 1974 (and like-design).
Early Sportsman Stock: 250 and 500cc classes for certain lightly-modified machines from the early part of the Sportsman era.
Sportsman: 125, 250 and 500cc classes for machines up to model year 1974.
Sportsman Open Twins: Twin-cylinder machines up to model year 1974, manufactured as 600cc and larger.
Open Age: Any AHRMA-eligible rider on an AHRMA-eligible vintage machine.
40+: Riders age 40 or older on any AHRMA-eligible vintage machine.
50+: Riders age 50 or older on any AHRMA-eligible vintage machine.
60+: Riders age 60 or older on any AHRMA-eligible vintage machine.
70+: Riders age 70 or older on any AHRMA-eligible vintage machine.
Women: Females riding any AHRMA-eligible vintage machine.
Post Vintage Motocross Classes
Note: All classes are further divided into Novice, Intermediate and Expert rider ability levels.
Historic 125: Certain 125cc and smaller motorcycles built up to and including the 1977 model year as the first-generation of long-travel machines.
Historic 250: Certain 126-250cc motorcycles built up to and including the 1977 model year as the first-generation of long-travel machines.
Historic 500: Certain 325-460cc two-strokes and four-strokes up to 636cc built up to and including the 1977 model year as the first-generation of long travel machines.
Historic Four-Stroke: Certain four-stroke machines built up to and including the 1978 model year with up to 636cc. Overhead cam machines are limited to 500cc.
Grand Prix 125: Certain post-1977 motorcycles up to 125cc.
Grand Prix 250: Certain post-1977 motorcycles 126-250cc.
Grand Prix 500: Certain post-1977 motorcycles 325-500cc two-stroke and up to 580cc four-stroke.
Ultima 125: Certain motorcycles up to 125cc that do not have disc brakes or power-valve type mechanism built directly into the engine.
Ultima 250: Certain 126-250cc motorcycles that do not have disc brakes or power-valve type mechanism built directly into the engine.
Ultima 500: Certain 325-580cc motorcycles that do not have disc brakes or power-valve type mechanism built directly into the engine.
Ultima Four-Stroke: Certain 200-580cc four-strokes that were produced either with a single-shock rear suspension or a newer engine technology.
Open Age: Any AHRMA-eligible rider on any eligible machine.
40+: Riders age 40 or older on any eligible machine.
50+: Riders age 50 or older on any eligible machine.
60+: Riders age 60 or older on any eligible machine.
70+: Riders age 70 or older on any eligible machine.
Women: Female riders on any eligible machine.