Hi @mikew2025,
Given the length of time that has elapsed since it got wet and that it wasn't cleaned properly as soon as possible after it got wet it may be beyond fixing. Also the battery may be beyond recovery as well but you can only try.
''The impurities in the water causes corrosion and provides circuit paths for the electricity which were not in the player's operating design and could damage the components. The corrosion starts immediately and is ongoing until it is completely cleaned away''
You need to dis-assemble the player, disconnect the battery and clean ''all the affected parts'' using ***Isopropyl Alcohol 99%+*** (available at electronics parts stores) to remove all traces of corrosion and water. If possible do not use "rubbing alcohol" as in some cases this is only 70% IPA or less, can contain additives and is not as effective. If you do have to use it check the label to verify the amount of IPA. The higher the percentage of IPA the better
Here is a link that describes the process. [[Electronics Water Damage]]
As always with electronics, especially surface mounted pcb be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board. Remove the shields that may be covering some components as the water may have got in under there and corrosion has started. The ends of any flex cables and their connectors need to be cleaned as well
Hopefully after you have done all this and the player ''might'' possibly work correctly again.
Here is the ifixit [guide|26498] guide that will help to gain access to the motherboard, battery etc.
Most probably you will have to replace the battery so use the information found on the battery to try and find suppliers.
Since you don't want to take it to a repair shop, this is the best that you can try.