Discontinued Grohe part

Need help with finding this part for my Grohe shower. Does anyone have a good source for discontinued Grohe parts? And if I can’t find a replacement what are my alternatives?

background: Reference the attached photo. In order to install a pressure valve, I need to remove the white stem (reference 1 in the image). Unfortunately, long term water leak had corroded the screw (reference 2), so I couldn't remove it from the white stem. It is now stripped, and I cannot remove it. I need a replacement for:
1) The white stem identified as (1) in the photo,
2) The screw, identified as (2) used to attach the stem (1) to the black base, and
3) The black square piece identified as (3) in the photo.
The current situation is that all three pieces cannot be separated due to the stripped screw (2).

 

This is a older Grohsafe valve. Many companies used this valve known as Tempress2.You will need to remove the 4 screws on the corners and remove the cover and cartridge. You will need to replace these items. I saw one on Amazon that costs less than the cartridge alone. RMV-1 is a Rohl part number but still the same valve.

New valve on Amazon

https://terrylove.com/forums/index.php?threads/newport-brass-tempress-ii-shower-faucet.89405/

 

Thank you! I will look this up. Appreciate the info!