[Dave at Pelican Parts]

I don't know all the rules, not by a long shot. What I do remember--

Some lightening of the car is allowed. The windshield can be removed. (Not any more I think, but a number of 914s have been grandfathered in because it was legal.) Plastic fenders are de rigeur.
Four 40mm throttle bodies. I think injection was made legal last year, but I'm not sure if injected cars have to use different size throttles.
Stock displacement, I think. There may have been an allowance for going slightly larger.
I believe cam and compression are "free", but I could be wrong.
Stock brakes, up until a few years ago when they allowed 914-6 brakes.
The cooling system can be modified; cooling air can be ducted from the headlight buckets to the cooling fan. (This rule was evidently written for British cars, but the air-cooled folks took it and ran with it. These cars have a "sewer pipe" running through the cockpit to take air from the front to the fan.)
Suspension is pretty free, but I think you need to keep the original pickup points and design. You may be able to fudge thepickup points slightly in some cases.
Wheels and tires are some spec maximum.
Umm... I forget the rest.

Most EP and FP cars run with very few blades on their cooling fan. Some run with none, just the four posts that hold the pulley for the alternator. They can (just) cool well enough with the ram-air effect when driving at speed that they do not need a full fan. Pushing all that air makes for a significant pumping loss, reducing your power output, so they ditch the blades.

--DD