SCCA street class accommodates aftermarket wheels with a few allowances... see section 13.4:
http://cdn.growassets.net/user_files/scca/downloads/000/019/208/2017-01-30-Solo-Rules-complete-reduced.pdf?1485811400
Quote:
Any type wheel may be used provided it complies with the following:
13.4 WHEELS
A. It is the same width as standard and as installed it does not have an offset more than ±7.00 mm (±0.275”) from a standard wheel for the car. The resultant change in track dimensions is allowed.
B. Wheel (rim) diameter may be increased or decreased 1” from the standard part.
Wheel spacers are permitted provided the resultant combination complies with the offset requirements of this Section. On vehicles supplied with an OE wheel spacer, the wheel spacer shall be considered as a part of the wheel. Wheel studs, lug nuts, valve stems (including pressure-relief types), and/or bolt length may be changed. Wheel bolts may be replaced with studs and nuts but the number of fasteners may not be changed. Tire pressure monitoring sensors may be removed.
Centerlock/Spline Drive/Knock-off type hubs may be converted to lug type hubs provided the resultant combination complies with the offset require- ments of this Section.
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With how many different size "standard" wheel options and sizes Porsche made available on these cars (although FYI I believe in stock class, you may be confined to looking at those available for your 2004 "S" spec), along with the +/- 1" wheel diameter and +/- 0.275" offset allowances on top of those, SCCA street class leaves some pretty significant leeway for legally changing our wheels!