Tim,
As you might have guessed, Porsche makes a special tool for this, called a "P7C." It's available from Baum Tools (Wayne doesn't sell them
AFAIK). This allows you to compress the spring on the car and remove and replace the spring and keeper. Fairly simple, but I wouldn't think you could do it with the engine in the car, maybe you could? The versions are different intake vs. exhaust, get both to be safe.
Another good idea is to get a HAZET 3428, this is a compressed air adaptor with a 14mm thread and a quick-disconnect, so you can hold the valve against the seat with air during the R&R process.
Act quickly: Lime Rock is 22 days away! Good luck!