2000 Ford KA cutting out when coming to a stop

Hi to all I am new to the this site, My brothe-in-law drove in to the back of some one not causing much damage to his car but has bent the bonnet, since this has happened every time he goes to stopping the car it cuts out most of the time, the car has had a new breather pipe and the spark plugs did have big gaps and were black which would indicate that it is running rich we cleaned them up re-gapped them to 28 and it is still cutting out, the engine has done 136,000 miles and the tappets are noisy but the car did run fine before the smash? I have had a visaul look and can see no wires or and damage to the wiring loom what else could it be and how can I test these out with out any machines to hand.

Please help as he has no money and is paying huge Insurance costs.

Jason Gawn