I purchased the Grand Am last July from my mechanic, who had replaced the engine with a newer one, (121000km). Back in December I was experiencing weird chugging and kind of hickup of the engine only while idlling. He said he could not find anything wrong with it. This continued until this past Saturday when it died completely. He has said that the timing chain went on it and that it was due to the fact that I was 3000km behind schedule on my oil change. I'd checked the oil level just the week prior and it was fine, though I knew I'd need to get it done very soon. He is also saying that the timing chain itself cannot be replaced, that the whole engine has to be replaced! Is this true? Also, does it sound as though the earlier issue could have been related to the timing chain, and he just didn't want to bother to open the engine to check?
Lack of oil changes will accelerate timing chain wear a little but not a significant amount. If that were the case all kinds of people would be sitting on the side of the road. (I haven't changed the oil in my '88 Grand Caravan in over ten years and 90,000 miles! That's abuse, not neglect, and I'm in no way suggesting anyone else do that, but it shows what some engines are capable of). Also, when a timing chain breaks or jumps a few teeth, or more commonly a timing belt breaks, you do not replace the entire engine. You repair it. There's two engines available for your car. Both use a chain and neither is listed as an "interference" engine. That means the open valves get hit and bent by the pistons as they coast to a stop. Some mechanics would recommend an entire used engine for that only because they're older, there's so many of them in the salvage yards, that they're cheap. Since yours is not an interference engine you just replace the chain, the sprockets, and any tensioning devices, and you're done. That is not a complicated repair. If your mechanic is not willing to do that, find a different shop.
Also, to identify there's some problem with the timing chain, the mechanic had to disassemble enough to see it. At that point he's part done with the entire repair and he should have seen it through. If he didn't take anything apart he's only guessing. I think you need a second opinion.