How can one make up for prayers missed for 26 years and Ramadan fasting missed for 20 years without a valid excuse, and is it obligatory to make up for the fasting by fasting, or is it permissible to feed the poor?

You must make sincere repentance for grave sins, and sincere repentance erases what came before it. As for missed prayers, there is a difference of opinion regarding the obligation to make them up, but making them up is more prudent. As for the days you missed fasting in Ramadan, you must make them all up as long as you are able. If you are unable, then you must feed the poor for each day, or your heirs can fast on your behalf. The amount of food is half a sa' of rice (approximately 1.5 kilograms), or one mudd (750 grams). And increase in seeking forgiveness and performing righteous deeds.