Neo backplate?
The EVO 120 screws into a backplate that fits underneath the motherboard. Two threaded pillars fit through holes in the motherboard, allowing the cooler above the motherboard to be screwed into the backplate below it, sandwiching the CPU, CPU socket, motherboard below it and a small part of the motherboard around it between the backplate and the cooler.
It is not possible to use the backplate provided with an EVO 120 (or other coolers) on a Socket939 (or 754) board because those backplates are for S775 or S478. The holes in the motherboard are in different positions on motherboards of different socket types. You would have to drill holes through the motherboard, which would ruin it.
It might be possible to physically fit the cooler without a backplate, by gluing it to the CPU. I wouldn't care to try, unless the motherboard was being mounted horizontally (a la desktop case) rather than vertically. The backplate is there for sound mechanical reasons. Without it, the cooler might cause sufficient torque to damage the CPU socket.
What I did was to obtain a K8 CPU mounting backplate (i.e. a backplate for Socket754 or 939) and used that. I got two, in fact. I bought one from Akasa (this is not normally possible, as they don't sell to the public) and I was given one at OcUK (this is also not normally possible, they just happened to have one available at the time from a dead motherboard) while I was waiting for the one from Akasa to arrve in the post.
It should be possible to use the K8 mounting backplate you can buy from Thermalright. You have to buy one separately - it is not supplied with Thermalright coolers (I checked an XP90). They sell them because Gigabyte S939 boards don't come with a standard K8 mounting backplate, so you can't use a standard cooler on them. Obviously, they're intended for people buying Thermalright coolers, but they seem to be a standard K8 mounting backplate (they would have to be, or you couldn't attach an XP90 or XP120 to them, as those coolers require a standard K8 mounting backplate).