with your engine mate just give it a bloody good service i had that on my 1.3 fezzi and after a real good oil change and engine flush she was back up to scratch...for some reason the 1.1 and 1.3 get blocked up real easy...give it a try!!! i also had that problem on my old 1.3 fiesta with the exhaust center hanging low...loosen then bracket behind the cat and get a mate to hold the cat in place the you will need to twist the pipe up till it right in and tighten the u clamp up it should sort it out...this might also be why your exhaust blows because just behind the cat on the joining part there is a small slit that becomes open when the exhaust aint in the correct position...i had all these thing and this cleared it right up!!!
in theory of course...if its that bad u can usually pick up a cheap 1.3 from the scrapy and they are so easy to fit in there...if you do go down this route your lookin at less about £100 max for the engine and about £20 to hire an engine crane for the day and just hope u got the right tools handy 
