I take it this isn't in UK but Spain / Canary Islands? Has it passed the local regs previously? If so what has changed that it now fails? Can you undo any change to get it passed again? Would it help if you could prove its the OEM exhaust that complied with EU regs when new?

Failing all those I'd search Ebay for a cheap baffle to fit, There's plenty of sizes available. I'd go for the biggest that will fit on the assumption that bigger is quieter.