'95 Prism Oxygen Sensor - threads

I've got a '95 Prism that blew off the Oxygen sensor from the exhaust pipe. The retaining nuts either fell off or corroded off. I can get a replacement sesor, but I don't know the nut size and the mounting bolts are welded posts to the pipe. Anyone know the nut size? Would I be able to spot weld a less-than-perfect nut on the posts to hold the sensor on? Any suggestions are welcome.

I saw a bunch of nuts and bolts for sale by size in bins at NAPA auto parts--maybe they would let you take a few out to parking lot to try them out.
Good luck.

Great suggestion. Thanks!