Grind down or machine down the diameter of the existing cog, removing the teeth, enough so a chain will clear the original cog. Lap the skip tooth cog on the outside of the original cog, hold it centered in a vice and drill holes through both cogs. The holes in the skip tooth cog may have to be bigger so it can be perfectly centered. Bolt the cogs together and try it to see if it clears the chain stays, that the chain line is good enough and to determine if it’s off center enough to cause the chain to bind. Once everything is good, weld the cogs together. I did this once about ten years ago. I used it for a short time but didn‘t like the gear ratio. I would advise against changing to a skip tooth, it’s pretty hard to get it just right, a lot of dissemble and readjustment before welding and you might end out with a bad chain line or not be able to clear the chain stays. You could ruin hard to find valuable parts.