If the cars done 200,000 miles and has a non genuine cat and a non genuine 02 sensor and no fault codes logged then I wouldn't waste too much time trying to fix it, I would be pretty sure that if it was a genuine fault then a Toyota would most likely Log a Code. ( And the fuel trims would be out )

If the Guy doesn't want to spend much on it then your on a hiding to nowhere. ??

other than a bottle of fuel treatment and thrash it to within an inch of its life to get the cat hot then the cars a Scrapper. ??

I get fed up listening to customers that tell us how little they want to spend on there cars but expect you to spend hours trying to bodge it and give them a liftime warranty when its done.