Is it permissible to fast the days intended to be made up before Ramadan, after it has ended due to illness and inability to fast?

You are not obligated to fast these days, neither before Ramadan nor after it. A mere intention does not necessitate anything, and an act of obedience only becomes obligatory through a vow (nadhr) that is expressed in words indicating commitment. As long as these words have not been uttered by you, your conscience is clear, and nothing is binding upon you. Your fasting is better for you if it does not cause harm.