Try to accept your current illness as your penance, since this is what God has permitted to befall you it is most acceptable to Him. Substitute some extra time in prayer. Remember, God wants mortification primarily of the heart.

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The penance I usually do cannot be done currently for health reasons. Simple question: Do I have  a right to avoid penance if it would cause damage? Should not penance be done whether one will be damaged by it or not, since extra suffering is a fruit of penance?

Why not offer a substitution?

St Thomas Aquinas - Summa Theologica


Whether the sorrow of contrition can be too great?

  Objection 1: It would seem that the sorrow of contrition cannot be too great. For no sorrow can be more immoderate than that which destroys its own subject. But the sorrow of contrition, if it be so great as to cause death or corruption of the body, is praiseworthy. For Anselm says (Orat. lii): "Would that such were the exuberance of my inmost soul, as to dry up the marrow of my body"; and Augustine [*De Contritione Cordis, work of an unknown author] confesses that "he deserves to blind his eyes with tears." Therefore the sorrow of contrition cannot be too great.

  Objection 2: Further, the sorrow of contrition results from the love of charity. But the love of charity cannot be too great. Neither, therefore, can the sorrow of contrition be too great.

  Objection 3: On the contrary, Every moral virtue is destroyed by excess and deficiency. But contrition is an act of a moral virtue, viz. penance, since it is a part of justice. Therefore sorrow for sins can be too great.

  I answer that, Contrition, as regards the sorrow in the reason, i.e. the displeasure, whereby the sin is displeasing through being an offense against God, cannot be too great; even as neither can the love of charity be too great, for when this is increased the aforesaid displeasure is increased also. But, as regards the sensible sorrow, contrition may be too great, even as outward affliction of the body may be too great. In all these things the rule should be the safeguarding of the subject, and of that general well-being which suffices for the fulfillment of one's duties; hence it is written (Rm. 12:1): "Let your sacrifice be reasonable [*Vulg.: 'Present your bodies . . . a reasonable sacrifice']."

  Reply to Objection 1: Anselm desired the marrow of his body to be dried up by the exuberance of his devotion, not as regards the natural humor, but as to his bodily desires and concupiscences. And, although Augustine acknowledged that he deserved to lose the use of his bodily eyes on account of his sins, because every sinner deserves not only eternal, but also temporal death, yet he did not wish his eyes to be blinded.

  Reply to Objection 2: This objection considers the sorrow which is in the reason: while the Third considers the sorrow of the sensitive part.

I read this recently:

What makes for a good form of mortification? You want to subdue the body, but you don't want to violate the dignity of the body. Anything that mutilates the body or causes serious or lasting harm is contrary to the Christian spirit of mortification, and contrary to the Christian vision of human dignity.

I think another aspect to this that makes it a bit difficult to know what's best is that a charitable priest or friend might discourage us from going too far, and they might be right to do so, but at the same time, we have countless examples of saints who seem to have sacrificed their health and their physical integrity.  My way of discerning for myself what's best is to focus on making little changes that I know I'll be able to keep up, and trying to do a little better every day, or week, or penitential season.