I have a Kusan battery-powered (one or two C cells - at work now and can't remember which), plastic wheeled/plastic chassis BEEP, which means it probably dates from the 1950s because Kusan went under shortly thereafter. The plastic body on my Kusan literally slips off that cheap toy chassis and onto a modern BEEP and appears to my eye to be from the identical mold. It was molded in green color and never painted, but otherwise looks the same as modern BEEP shells.
Most of the molds used to cast modern locos have been passed down from one failed company to others and this is the case here. It went from Kusan to someone else to Williams or someone to RMT. I don't know whether Marx had this modl before or after Kusan or it is actually never did. A company called AMT had some molds Kusan used before Kusan bought them (late '40s, early '50s).
Regardless, RMT has it now and a really solid two-motor chassis. I agree that the BEEP body is very "toylike" but it looks pretty good painted and dressed up as RMT does it now. Moreoever, it is amazingly similar to the Brookville BL06, a locomotive that was designed and built decades after the Kusan BEEP, shown below
