Lots of posts on chain stretch - try a search. Basically you line up the marks on the cam and cam bearing tower and observe the timing marks on the balancer.
You need a 27mm socket to put on the balancer bolt to turn the engine over
If you get a worse chain rattle at start-up, you need a new tensioner. You should attend to this, and the broken rail, sooner rather than later.
for body parts.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe