That’s nuts. It probably runs like trash without a sensor to close loop on.
If you pull the downpipe any welder can MIG or TIG one on. There are also clamp on bungs but they are kind of dodgy looking.
This is the last one I did. I’m a poor welder but I can make an O2 bung stay on a pipe. It doesn’t require quality aerospace welding. I buy my sensor bung and plugs on Amazon. They’re pretty cheap like $10 or so. If you have a pal with a welder it costs a six pack.
ETA - yeah running rich sounds right. Your ECU is reading 0v on the sensor because there is nothing there. It thinks it’s lean. So it injects extra fuel till it sees 1v(rich) which it never does and thus doesn’t back off and you get your misfire condition.