My take would probably be:
a. If you find other old or strange spellings, leave this as "ancle", on the assumption that this is the original and it's not wrong.
b. Otherwise (or if there are other instances of "ankle"), change it to "ankle" without notice (unless you give a full, detailed list of changes), based on the existence of contemporary editions with this spelling, which may indicate that this was actually a typo and unintended by the author.
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