91 240SX Timing Chain Replacement
 

Looking for some advice/guidance on replacing the timing chain in a 91 240 SX w/ a KA 24 DE 2.4L DOHC engine.
I have been repairing american and foreign cars for approx 50 years but have never been into one of these engines. Car belongs to a friend. It is a beautiful car for a 91, almost showroom condition. 140,000 miles, runs great. Shop told him timing chain is rattling and will cost $1600 to repair. He ain't got that kind of money and asked me to look at it. I couldn't hear much of any noise although hearing isn't what it used to be.
Questions:
How serious a problem is this?
How can I check or confirm the problem?
Can the chain and tensioner be replaced w/o pulling the head?
Any other hints/advice would be appreciated.
Thanks,
John