Cut off you old lug keep it if it is long enough or scrounge another piece of scrap steel about 1/8 in thick. Take your new lug and hack saw the top forming a 1/8 in square by 3/16 tall spigot. File this round, drill a 1/8 in dia hole in the guard where you need your new lug to be. Countersink the top of the hole about 1/32. When you drill and file everything up use no oil and do not touch any of your "machined"surfaces. Now flux the spigot and hole. Grip the lug in the vise with the spigot up. Set the guard on the spigot and peen until you fill the countersink riviting the lug and guard together. Now inspect, and solder. Done, takes less time than it took me to two finger type this.
P.S. Soft solder works just fine