Have you down loaded the -24P TM?
The part hanging there at the bottom is the three way valve, but more then that I can not say. Need better pictures.
The three way valve hook up should look very much like this picture below. This picture is not for your gen set, nor the MEP-004A, with is 98% the same as the MEP-005A. This is a picture of the MEP-006A. But should be almost the same as yours.
Notice, that the Three way valve has three outlets, (makes sense!) One outlet goes to the Auxiliary Fuel Inlet nipple, (outlet to the right side of the valve) under the valve, when its mounted to the side of the gen set with two bolts/nuts. Another outlet goes to the fuel tank, (Outlet to the left of the valve). The third goes to the gen set fuel pump system, (outlet going straight up).
The below picture is for your set, to help explain what I just wrote above. The three way valve is call "Fuel Selector Valve in the picture below.
The first thing you need to do is look for the ends of these three connections, and see if they are laying inside the set. Or, if someone took them. The belly of a gen set is a graveyard of missing parts, so take your time and look good. The parts can be located between the side of the fuel tank and the gen set housing. They can also be in the OVM box there, under the dip stick. You are also going to need the two mounting bolts/nuts. Lets see some more/better pictures. Balls in your court.
Also would not hurt to take some good pictures of all the rest of the set. Might be able to see something else missing or broke.