I'm a little confused as well. On an M103 the guide rail pin only needs to come out if you're pulling the head. But, you're talking about an M104, which I'm completely unfamiliar with.
The piece of plastic merely gives the cover something smooth and rigid to slide on while being pushed down. I'm pretty sure it wouldn't matter what kind of engine it is, but whether it helps with that pin issue, I have no idea.

While I'm thinking of it, assuming a pin is in the way, and you remove it to facilitate putting the cover on, how do you put it back in once the cover is on?