Been there and done that, and I'll try to describe my repair best as I can. If I can find a photo of that job I'll post it later.
First I cut off the remaining stub with a pneumatic cut-off tool. There will still be a raised round spot on the frame.
I center punched that round area and drilled it out to whatever diameter that is for the bolt you need for the nut that holds the shock on. Note that I drilled in several stages, using progressively larger bits.
I inserted a heavy grade bolt of the proper diameter from the inside of the frame, poking out. I placed a bushing/sleeve over bolt in order to create that larger diameter smooth surface. The inside of the sleeve fits the diameter of bolt and the outside diameter is the proper size to fit the hole in the end of the shock. I think I had to cut the sleeve to length but the ID and OD required are an off the shelf item. Use the regular nuts and washers to put the shocks back on. I also suggest a little anti-seize compound on the threads.
I did this maybe six years and thousands of miles ago or more. The ride is good, shocks are still on, and there is no apparent weakening of the frame. If you have a welder, or know someone who does and they will do the job for you, once you got the bolt in place I suppose they could weld it to make the joint solid metal, but given my results that is probably overkill.