1031 to old rental

I am looking at a short term capital gain on a property I am selling. Can I use 1031 to replace the property with one I already own by paying off the mtg.?

No. You can’t 1031 into a property you already own, only into a new property that you will acquire.

Even if you locate an acceptable replacement property, there may be some question as to whether the property you are selling would even be eligible to participate in a 1031 exchange in the first place.

We don’t have enough information to be any more definitive.