What is the expiation for adultery for a married person who repented 20 years ago, and is it permissible to offer the expiation of feeding instead of fasting due to inability to fast?

The expiation for fornication (zina) is sincere repentance. However, if fornication occurs in Ramadan, then both making up the day (qada') and expiation (kaffarah) are required, similar to the expiation for ẓihār: freeing a slave, and if that is not possible, then fasting for two consecutive months, and if that is not possible, then feeding sixty poor people. If you are able to fast, it is not permissible to move on to feeding. You can use equivocation by saying that you are fasting because you had a discharge one day in Ramadan, and that you are adopting the Maliki school's view as a precaution. The expiation is binding if you were an adult at the time of the fornication.