I just replaced the same fitting on my water heater. The piping is PEX and I cut mine back where there was room and installed a PEX x Male adapter. Then I connected a stainless steel faucet supply flex 1/2” female x 1/2” female to connect between the water heater and the newly installed PEX x MIP. It makes disconnecting of the water heater much simpler and eliminates the fragile plastic 1/2 PEX x FIP 90. I am sure I will be replacing more of them. There are a number of different ways to connect PEX (crimp rings, sharkbites, expansion tools, etc) but I have crimp ring tools (I am a plumber by trade) so that was simplest.
One other note-be careful with the MIP threads on the water heater, mine really wanted to cross thread. Good Luck!
1999 Maxim 3000 SCR
(Twin 5.0ís Mercs)