Fouled plug will generally not miss and rev up smoothly. It will idle lumpily and rev up rough. But it that is what you mean then checking the plugs is a good start. You'd need to check into plugs for that model.
At close to 30yrs old a miss these days can be all sorts of things. Carb, wires, coils, CDI box there is quite a bit to check and you need to determine if fuel or spark.
Says it happened when he did the plugs and fuel line. If you think he is being truthfull, start with where he says the problem occured, but putting the tank back on he could have hit. pulled or kinked either a spark or fuel piece.
Good luck, take your time and try one thing at a time. A hanging fuel tank can also help ease the work and eliminate the kink concern.