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Been doing a bunch of reading, and it LOOKS like that 968 rims and some Boxter rims are a straight bolt on situation for a 1989 S2? I plan summer tires on the new rims, and winter tires on the OEM rims.
If you are considering 18" wheels, I have a set of Porsche hollow spoke technology wheels with center caps that are a direct bolt-on for the S2. They are 7.5x18-ET50 and 10.0x18-ET65. Here's a picture on my S2. I purchased them new; they have about 32K miles and have been in storage since 2008. Wheel wts are 20.3 lb front, 24.2 lb rear.
I had chromed Design 90s when I bought the car last year. I had them refinished but the chrome had corroded so they leaked. I took a leap of faith (and checkbook) and bought 17" Fuchs. I think they make the car, but that's just me. Good luck!