The anti siphon valve is usually screwed onto the right angle fitting as it comes out of the gas tank headed forward toward the engine. It will usually be threaded on one end and barbed on the other to accept the fuel line. The fuel shut off valve sounds "aftermarket" and is probably not necessary, possibly a cause of the problem depending on the valve???
Google image - "anti siphon valve fuel tank" Many images of the valve and location will come up.
This is another trouble shooting suggestion - http://www.justanswer.com/boat/6z9vs...-performs.html
The pump changing sounds definitely sounds like an increasing restriction. Could be a hose getting soft and collapsing too. Our fuel lines are pre-ethanol so this may be taking its toll as well. As the link suggests a vacuum gauge may be your trouble shooting friend as well.
My anti-siphon restriction got progressively worse over the course of a season. When it started, I could run for 10min+ or longer then start losing power, then less time and less power. I threw carb rebuild (found a cracked float anyway), fuel pump etc at it til I finally got a vacuum gauge.