You can have the wallet since you find it but you can never own the money/coins inside since you don't have the keys that will open it. Therefore, it was useless to find it, you don't have any benefits as you can never use that wallet. If I were you, better make your own wallet to have full access rather than claiming that the found wallet is yours. You can legally claim it was yours when you have the keys that is the way to prove ownership. Otherwise, cour claim id invalid and just a waste of time as we can never get anything that is inside.
OP is discussing about obtaining the private key or seed phrase and suddenly finding Bitcoin inside.
If I miraculously got a private key to a wallet that had Bitcoin in it
To OP, technically speaking, if you found Bitcoin within a wallet to which you have access by obtaining a private key, you can do whatever you please with those funds. However, if you have a conscience and are aware that the funds are not rightfully yours, you would likely choose not to use the wallet and instead turn a blind eye to the funds.