I've got a mgzs 1

Hi I've got a mgzs 1.6 facelift 05 I had to change my head gasket changed the cam belt and got a uprated head gasket on. The car waz running fine till a couple of weeks ago when I noticed car starting to misfire then the engine light came on so took it down to the garage where they put it on a diynostic machine and it came up with a injector problem changed them aswell as spark plugs and checked ht leads they were fine but it still hesitant when driving and misfiring so rechecked timing belt it was fine because it waz running fine before the problem. But I've been doing a bit of research and similar problem point to the cat converter when it's blocked how do you think I could resolve this or could you shed some light on the problem I'm scratching my head
Cheers
Derek