Must one re-observe the Ramadan fast if he experienced a wet dream before the beginning of the month and was unable to perform ghusl due to surgery, sufficing instead with wiping the hairy areas?

Firstly: It was obligatory for you to perform tayammum instead of ghusl when you were unable to do so, as it is legislated for both major and minor ritual impurity.

Secondly: Purity is not a condition for the validity of fasting. The Prophet, peace and blessings be upon him, would be overtaken by dawn while in a state of janabah (major ritual impurity) and would fast.

Thirdly: You committed a great error by praying without purity, as purity is a condition for the validity of prayer by scholarly consensus. One is excused for this due to ignorance or misinterpretation, and you are not required to make up the missed prayers.