Mr H is correct, some of the early 993RS engines were blessed with the 964 vernier cam gears but most had the later one-piece friction gear. Interestingly, some of the engines with the friction gears fitted also had camshafts compatible with the verniers, but the only way of knowing this is by taking them apart. I should also mention that most of the camshafts from the later engines had the end of the cam reduced in diameter to ease the fitting of the chain gear and finally some cams did not have the woodruff key slot cut into the end, so the worst case scenario is if you have a late cam with no key slot and turned ends. As Paul said, we found his engine to have these features and had to do some machining of his cams to reinstate the full location diameter (essential to support the bore of the vernier chainwheel) as well as machining in a new keyway.
Changing the cams can be done with the engine left in the car, from memory it will take around 12 hours to remove them and refit & time them up (using proper dial gauges, not the poxy Porsche alignment tools), so I would expect the total bill to be in the region of £1000+VAT including labour, gaskets, oil, machining and gears.
The only other common problem with these engines are the hydraulic cam followers, so obviously if you have a high mile engine and are doing the cam gears, it makes sense to fit a new set or convert to solid lifters whilst you have the followers removed.
