if it was me in this suituation i would found out the law with the tuner/garage being covered by public liability damage for any work they undertake that goes wrong or any damage that occurs.

Then have the engine inspected by independant engineering firm and a report filled out on condition of the rest of the engine and probable reason for engine failure. Present the report to garage who were fitting the Nos kit and supposed to set it up and see what they are willing to do about it. (as from info given it does seem t be their fault)

If not then its solictor and small claims court, but you would have to pay out for the engine rebuild before you could try and claim it back