Personally, I wouldn’t ignore it as it looks bad on your part if he really has suffered a failure and he gets a lawyer involved.
personally I would respond politely in writing stating that the car was inspected by his own engineer confirming that, at the time of the sale, there were no obvious faults or evidence that a failure was imminent. As a result of this coupled with the fact that the car was sold privately and came with no guarantees means that you will not participate in a cost sharing exercise.
stress that it is unfortunate for him and you are sympathetic to his situation, however you were not aware of any mechanical issue with the engine at the point of sale and as a result do not feel responsible in any way.
tell him you won’t be involved in any further discussion or correspondence relating to this matter.
write the letter in a way that you would be happy to take to court, that way he will have to spend money to peruse you, if he’s skint he probably won’t do that anyway as his chances of success are very small (unless there’s something that you aren’t telling us)
this is what I would do to both close it down or show myself in a good light if it went further.
If you get something formal from the court or a solicitor, take advice. If you end up with a broken leg, go to hospital.