Hello Dan.

I guess you are doing great.

That shouldn't be a problem, you can carry the same original Second PUC marks card for your KCET verification. In case if the stain is on some essential information, you can also carry the Xerox copy of your original marks card which has a clear information. But don't forget to carry the Original as it is must.

Also you can get your Second PUC marks card on DIGI LOCKER. Even you can use that for the reference.

I also suggest you to get a duplicate copy if the stain is too disturbing.

Hope this was of some help.
All the Best!!!