I don't know about putting in a fuel shutoff but I have added those pancake filters in these lines. I got group at a nursery that is always getting trash in the fuel tanks. Yes it is a tight fit. The o-ring are oblivious sealing ones. Double check that module is clean as it does sounds like you still have a clog of some kinda. You don't need to take the carburetor off to check for fuel as there is a drain plug in the bowl as found that easier for getting water drained out.
Now Luffydog there is no way you can deny being a mechanic. Like a wife with dishpan hands.
Sorry just had to say it.
One suggestion that helps me on those pins as my big fingers have very little grip due worn finger ridges is to try a pair of needle nose pliers to get a grip on that pin. Meanwhile I go put my waders on as it is pouring down here so I can get to shop. Now I know why I found a pair of hip boots in the barn.
The truest measure of society is the how it treats its elderly, its pets, and its prisoners.