It's really easy:
Put the lever without mount anywhere in your map and just the mount where you need it. Make a script area that fits the mount. Then;
void OnStart()
{
AddEntityCollideCallback("Lever_without_mount", "Script_area_mount", "Mount", true, 1);
}
void Mount (string &in asParent, string &in asChild ,int alState)
{
SetEntityActive("Lever_mounted", true); //This entity is the normal lever
SetEntityActive("Lever_mount", false); //Mount of the lever
SetEntityActive("Lever_without_mount", false);
}