What part do I need to repair? My toilet water tank top/lif just suddenly flew off and water was spouting out. I noticed the inside parts/kit is a fluidmaster 400 series non-siphon...

Well, without knowing exactly what caused the issue, or what may have broken to allow the top to pop off, I would suggest simply going to a hardware store and purchasing a complete new fluidmaster 400A fill valve. Fairly cheap and easy to replace the complete fill valve. In this way you will know the problem is fixed. Take care and let me know how things work out.

Chances are it just popped off... Happens all the time... If that's the case, than it is quite an easy fix... That cap sits on the fill valve by being screwed about 1/4 turn in. Try to reinstall it. First, flip the cap over and see the pegs along the perimeter of the cap. Then, align cap (pegs) with the top of the valve, place the cap on top ( feel it to slip over the valve in its place... ), apply a bit of pressure straight down - and turn. That should do. Let me know how you did. Back to you. Milo