There are four screws holding the shell to the sheet metal frame. Turn the loco over upside down, turn the motor truck to one side and that will make it easier to access the screw.
When you remove the four screws, you carefully work the shell backwards, to remove the handrails out of the shell cab, where they insert into a small hole.
I have no idea though, how you are going to make a MARX coupler fit into the K-Line truck without some serious modification. K-Line completely redesigned the trucks and they are not similar to the original MARX version of the S-2. Maybe some MARX fan has some experience with this one.
It would probably be much easier to replace the S-2 coupler with the correct K-Line one. Brasseur's got most of the K-Line parts when K-Line folded, but I don't see the coupler listed for either the S-2 or the Alco FA. They have a complete coupler for the MP-15, which will be a little longer, but will work.
http://www.brasseurelectrictra...vice/kline/kline.php
An easier idea would be to make a transition MARX car by drilling out one of the MARX trucks and replacing it with a Lionel, or K-Line truck, and then run this car first in a train, pulling the other cars with the MARX type coupler.