I've adapted Marx E7 diesels to run with Lionel trains. They're good runners, as good as postwar Lionel Geeps. First thing I did was add an E unit lockout switch to avoid inadvertent reversals.
The Rock Island unit shows how I added a Lionel coupler to an E7B diesel. Added an 0.40 filler strip beneath the truck frame and reattached the coupler bracket with a 2-56 x 3/8" machine screw. I attached a Lionel dummy coupler to the Marx tab coupler with a 2-56 x 3/8" machine screw. The tab needs to be drilled 3/32" for this to work. There are two tab lengths on Marx diesels. The longer tab length works better.