This is a serious ethical question, for which there is more than one answer. You should make restitution, just as you suspect. How much value does the object have? Have you worn it out perhaps--or used it up? How has the person from which you got it coped? (Did they buy another, figure they'd lost it, report it to the police, what?) And, by illegal, do you mean that you stole it? I'm not certain of that.

Do you see how many options present themselves?  Here is the guiding principle. Your satisfaction or restitution should accomplish that purpose.  If your obtaining the thing illegally harmed someone, that's what you need to focus on--repairing the harm. Sometimes, you and your conscience were what was harmed most. Your best manner of restitution could be to give an equivalent value to a charity, not necessarily to the person from whom the object was acquired. Because God sees all, God knows your purpose is to make restitution.

Pray about this. I believe that your conscience will guide you in the best course of action. If you still feel doubtful, consult a priest in confession.