Is it permissible to help one's father tend to a farm that he bought with an interest-based loan, and if helping is not permissible, is it permissible to work with him for wages?

There is no harm in helping your father cultivate his land and working with him on it, whether for pay or as a donation; for the prohibition of usurious loans is tied to his liability, not to the specific item for which the money was consumed. He must perform sincere repentance and rid himself of the loan if possible, so that the usurious interest is dropped or reduced.