SKS scope base and mount
Since it is a top ejector, about the only thing that I have found is to mount an SKS scope base and scope on the back of the receiver. I have one mounted on a Model 1911 which was the predecessor to the K31. You will be limited to about 150 yd shots, but with mine in place, it will stack them in a circle the size of a quarter all day long with this combination.
I had a longer scope mounted on it at one time, but if you had to cycle the gun in a hurry, the empty would ding on the bottom of the scope. Not too good if you reload, but if you don't, it won't hurt anything.