Don't know the original factory Porsche specs for either the 8mm or 9mm stems, but.....
Kolbenschmidt TRW list the cotters for the 9mm stems (MK-9H) as non clamping design (I'm assume using their valves as well) - there may be some useful dimensions on the cotter/valve stem drawings for you to compare with.
As for taper, you can measure the retainer using 2 different sized ball bearings, a vernier and some trig. calcs.
KS TRW Valve Cotters
KS TRW Porsche Valves