Hints for timing chain replacements on OM617:
1. Bring a friend. Best move you can make. You need two pairs of hands, one pair to feed the new chain and keep tension on the old chain as it comes out of the hole, and one pair to turn the crankshaft round and round until you're done.
2. Forget tiewraps, they break. Forget bread wire ties (don't laugh, I've heard of them being used). They break too. Small needle nose vise grips can be used, either with minimum tension directly on the chain and sprocket, or with rags protecting the sprocket and chain. I used them without the rags and just enough tension to hold them in place and they worked fine.
3. You can do the whole thing without taking the glow plugs out. It's just like turning the engine over during a valve adjustment. (Does anybody pull gp's for that?)
4. Stuff lots of rags around the open whole below the cam sprocket when you grind off the link. If you use a die grinder instead of a dremel, think very carefully about where the wheel is at all times.
40 isn't too old to do a timing chain change. It's really not too complicated, it's just that the consequences of a mistake are very high. Drop the chain, you're pulling the front cover. Drop the link plates, you're pulling the front cover, etc....