Why not get it replaced, if it is still under warranty?
If the solenoid threads on the valve are 3/4 x 20, then you could probably use an Irritrol latching solenoid to actuate it.
I'm in Turkey, and warranty is valid only for 2 years.. This devices are about 5 yrs old.
Anyway, I just changed the capacitor inside with a 16V 10.000uF instead of 8200 uF (because i got only that at home) and used a spring taken out from a pen..And now it works..