Water leaves lots of nasty stuff behind which causes corrosion over time. Think about a car battery and how the terminals grow green stuff over time. Without a proper water damage recovery service from a shop you will have long lasting effects from the water damage. Chances are that this was so long ago that the corrosion has caused permanent damage to either the connector where the LCD and Digitizer attach or the touch IC has been compromised. Take it to a shop and have it cleaned properly which includes removing the EMI shields, and possible replacing a few components.
I have worked on a few of these cameras and getting parts is almost impossible. If you find them it will probably be on EBAY from a unit that is no longer functional. It would probably be more time effective and cheaper in the long run to just send it to Nikon for service. Taking this thing apart and getting to the mirror assembly is a 10/10 difficulty. Replacing the mirror is not for the inexperienced.
Here's whats most likely really happening. The parts in the after market are not all manufactured by LG and Sharp like the Chinese Distributors would like to let you believe. They are largely counterfeits. OEM and Original are just words they have chosen to describe parts that are very near Apple's required quality standards but these were or would be rejected by the OEM. There are 3 things at play in the market. 1. An oversupply of the really crappy parts. This is because repair shop owners and american distributors have been pushing the Chinese suppliers to deliver higher quality. Nobody is buying the low quality and thus its just siting around. The Chinese find a way to sell and make money on everything. 2. Apple wants people upgrading to the 6. The device life-cycle is only meant to be about 2 or 3 years. This is when Apple wants people to transition off the old device to a new one. This is how the entire electronics industry works. They have been reducing the lifespan on the devices in order to...
Chip could be going bad, thermal paste under the chip cap hardened and went bad, or not enough airflow from the fan. You can mod the case to improve airflow. There are videos on youtube. Other solutions will require a visit to the repair shop.