If you obtained approval to remove the air conditioning units, then it must have been considered by the Committee at the very least. What did they say? From what you have said it is likely they have assumed you will be bearing the expense. It also sounds like you are removing the air conditioners and reinstating the windows, is that correct, is that what you have received approval to do?
If it was a bad DIY job it is highly likely that the person who did it didn’t get permission. If they did get permission, then as Jimmy says there may be something in the by-laws or the records indicating who is responsible. If no permission was given then it is arguable that you could require the OC to reinstate the common property (assuming the windows are common property).
However, you bought the property knowing that the hideous installations were there, and I suggest it would not be a wise move to say to the OC that they should pay for what you are doing. On the plus side, if it is an investment property you can include it in your tax.