If I'm right about it being a 400, yours is like a high school fling, rode hard put up wet, and needs a lot of work. Reason I'd strip it for parts is because generally speaking I wouldn't trust someone else's welds done at home, amongst other issues. Plain and simple it was probably someone in the learning stages of general mechanics and lots of corners probably cut.
As far as building a fl350 type cage, well, it's a tiny frame to begin with, meant for a tiny engine and tiny rider. Build the cage just slightly too low and you may only get a week of riding it before you out grow it. I do agree, take all the wheels, steering parts, and whatever else is actual go kart parts off, sell the frame for the $50 you have into it, and find a larger frame that hasn't been tortured all it's life to build. A roll cage is a blessing and a curse, if you are strapped in with a full cage you may be safer in a rollover, but harder to get in and out, and the cage may collapse in a bad enough wreck to pin you inside. No roll cage should also be a no seatbelt kart, that way you are thrown out of the kart, potentially out of harm's way, I should practice what I preach, but wear a helmet ALWAYS, regardless of weather a cage is present or not.
The picture is the little 400 I did for my niece and nephew, there isn't much to them and easy to build, but if you build yours look it over very carefully and be comfortable in the understanding that your inspection could be the only thing keeping you from serious injury, or yes, even death. Your call ultimately, I don't have to drive it!