Carsnob
There is an excellent faq on cylinder heads and pistons on the 2002 website.
What little is known is the cs designation changes things; most likely chamber volume and pistons. Since you presumably know the oversize by measuring, I suggest you measure the chamber volume, compare it to the charts in the faq. Perhaps post a photo or two of the head and the piston shape you have for further discussion. Look carefully for any markings, letters, etc. on the head of any kind.( Just in case it's an uber rare Alpina head with 4 letter designation- stranger things have happened)
I second Chris's observation, most likely 121 since it started in 68. But in the world of classics, any number of things could have changed. I too suggest Ireland due to the fact they can source custom pistons at about the same price with the desired deck and shape to either maintain the same hp,. correct it, or increase it. Hey- 20 hp is 20 free hp in my book.
Last but not least, there are some members here who own a 2000c or cs. There are quite a few 02 guys here too. A little searching and a pm is suggested.