Here's a starting point, based on the MWS rules. Feel free to <copy> and <paste> to move things from one category to the other.
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GENERALLY ACCEPTED RULES FOR THE 1/8 GT CLASS
chassis: any 1/8 scale buggy based shaft driven car
diffs: standard gear diffs only
body: any commercially available GT style body
body: rear of body can't be completely cut out
wings and spoilers: allowed
tires: any commercially available rubber tires (no foam tires)
tire inserts: any insert is OK
tires: must fit completely under the car body (no add-on wheel flares)
stability: no electronic stability enhancers
engine: any .28 or smaller "spec" style engine.
engine: must be commercially available
engine: must be available from mail order shops for under $200 w/o tax
clutch: non-adjustable 2, 3, or 4 shoe clutch (no axial clutches)
pipe: any pipe
UNRESOLVED RULES FOR THE 1/8 GT CLASS
weight: minimum weight 3500 grams
fuel capacity: 150 cc