If he put that much $ into the car if you can find the problem you should have a good car.
Did he ever take it to an auto diagnostic place?
Engine misses are caused by basic things; wrong fuel mixture (covers air/fuel), wrong timing, no or poor ingition spark and poor compression. What can cause those 4 things to be incorrect can be complicated.
In the old days you were able to check all of these as there was no computer on the car to deal with (and no electronic emission controls either); but now trouble shooting can be a nightmare.
The eingine diagnostic analyzers (that they have in a shop not a hand held one) they have today can even tell you if the compression is bad (they measure the resistance of the spark jumping across the gap high compression = high resistance).
Of course it is your choice. I would at least call a diagnostic center and see what they charge.
Best wishes.