What should I do about the money I found and spent, given that it was a lost item which I did not publicize?

The summary is that a "luqtah" (found item) that must be announced is one that its owner would follow up on and that has value. As for something trivial or worthless, it is permissible to possess and benefit from it without announcing it. The criterion for determining what is trivial is custom. If you benefited from money without announcing it and years have passed, then you must announce it now if possible. Otherwise, give a similar amount in charity on behalf of its owner. If the owner approves of the charity, they receive its reward; otherwise, you must pay them back, and you will receive the reward for the charity.