1991 Nissan 300ZX Timing/bent valves

A little over a era ago my timing belt and water pump were changed. About one month later the water pump failed and the mechanic replaced the water pump and replaced the timing belt again. The car ran fine for about a year, then the air flow sensor went out and at the time (same mechanic) noticed some pinging. He mentioned there was a special tool he had to buy to adjust the timing without tearing apart the front end of the engine. About a week later the car suddenly stopped, wouldn't start. The mechanic called and had me come down and look at the car. He showed me the timing belt was tight, the two cams were set perfect but the crankshaft gear was off by about 60 degrees. The tensioner was also fine. He said the only thing he could think of why this would happen (with all parts ok) was a sudden jolt to the engine. The belt was fine and no cogs were broke. Obviously this condition has bent the valves and now a new engine is needed. Have you heard of this? Is there a tool that can be used on a 300zx to adjust the tensioner or timing. My brother who knows more about cars than I do, thinks if the tensioner slowly went bad or broke, then the belt would loosen and cause the problem. But then the belt wood be loose. I don't think the mechanic would hide anything, he has done alot of work for my family. My only redflag is the problem a year ago and the adjust I was told happened the week before. Thanks for your help.