There's a few of us here that do this. I've been reading and watching what's posted. It is hard to get the fixed pilots from MTH. When you can, I believe they're like $25 each. Then the railings and other parts I've never been able to get.
So most of us adapt the pilots I believe. It is so much easier to get the two rail version. Sometimes there isn't any ever made. All their dummies are only available in three rail for example.
I've just made a shim like Jeff describes to fill the gap. Each engine has a different challenge but mainly the same proceedure. Get the six wheel set and make fillers, mount some KD's and fill the gaps if desired. The railings I still haven't fixed myself.