As a rough guide, if your car has modifications beyond this list it is not eligible for Stock Category:
-Anything available on the exact model and year of your car as standard or optional equipment, as installed on the factory assembly line (nothing available only dealer-installed is allowed), plus
-Standard OEM identical replacement parts
-Accessories, gauges, appearance, comfort, and convenience items (not replacement driver's seat, though) that basically don't help
-Added roll bar or roll cage to spec (not required, though)
-Driver harness (no cutting seats to install, though)
-Trailer hitch and/or tow bar mountings
-Any wheel of diameter and width identical to a stock or factory optional wheel, offset within +- 0.25 inch
-Most street legal tires that will fit the mounted wheels and stock bodywork
-Any shock absorber of the same type and mounting as original; no change to standard spring mountings is allowed; suspension geometry may not be altered; gas or hydraulic shocks are permitted
-Any brake linings; pre-1992 cars may use solid/braided metal brake lines.
-Any front anti-roll bar (no change to stock/optional rear anti-roll bar, if present)
-Manufacturer specified ignition settings only
-Any replacement air filter element (or removal of air filter element); no plumbing changes, however
-Engine bored no more than .020"; no balancing or porting/polishing of head
-Replacement of any part of the exhaust system past the catalytic converter (if quiet)
-Any oil filter
-Added clutch scattershield
See here for everything
http://mouton.best.vwh.net/sccasolo/Rules/stock.html#s131
)