Old Richdel valve stuck open?

https://preview.redd.it/kptv2jxrc7jh1.png?width=973&format=png&auto=webp&s=70d2cd1399debb2097f23260663b7aac3ce48efb

Hi all, I believe this is an very old Richdel valve, it can be powered and I can hear the solenoid humming, but it is stuck in the open position and the sprinkler won't shut off.

1. Is there an easy way to flush the inner without taking it apart?

2. Is the diaphragm even available at this point?

3. Are there drop in replacement that I can retrofit into it without having to replace the whole valve?

Thanks in advance.