If my 2 cars are a judge, at 253K miles, your timing chain has barely worn. All you might need is to install an offset key in the upper sprocket, and probably the smallest one (2 deg = 4 deg at crank). That is a cheap part and a fairly quick and easy job, next time you are having the valves adjusted. But, measure first so you buy the right key (here on P-P). And don't let a gomer mechanic touch it. A smart guy will stuff rags in the chain cavity so the key isn't dropped, use a magnet, use bungie cords and wire to secure the sprocket to chain relation and not let the chain loosen and bunch up, or miss a tooth down below, and other obvious care.

I wouldn't worry about valve guides. As long as the valve stems aren't wobbly, there is no concern. Unlike gas cars, there is no intake vacuum to suck oil past the guides, and even if it did the oil would just burn like diesel does. Indeed, I have heard of rings so worn that oil past the rings causes the engine to keep running on that, though may be apocryphal stories.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans