How to fix Watts anti-siphon valve 800m4?

Question:My dear Father, whom I love, though incredibly bright occasionally mistakes rather stupid mistakes. He was trying to get his boat up and running and used one of the two faucets on the side of our house. Unfortunatley, the one he choose is actually the anti-siphon valve to our sprinkler system. Long story short, he blew the cap off of the top and now our sprinklers dont work. We've tried buying a replacement cap (from rainbird) but it didnt match. Now we are not sure what to do short of buying a whole new valve. Its says Watts Regulator on the side and its the 800m4 series. THIS {http://www.plumbersurplus.com/prod/watts... is what it pretyy much looks like, and the black thing on top is what we are missing. We really need to get this thing fixed so any help would be appriciated. if you need anymore information please email me. Thank you.

Answers:
There is a kit that includes the cover. You will have to go to a sprinkler supply house to get it. Just look up sprinkler supply in the yellow pages. We keep them stocked on the work truck because this does happen.