Is it permissible to compensate for feeding sixty needy people with money or other charity, given the difficulty of finding them in one place, after I made that intention for my success in exams?

Collecting sixty poor people to feed does not require them to be at one time or in one place. If your intention is merely charity without a vow or a binding formula, then you are not obliged to fulfill it, although it is among the virtuous acts of obedience. However, if you have committed to a vow for the sake of Allah, then you must fulfill the vow by feeding sixty poor people if you are able, and it is not required to feed them at one time.