Unless your timing is irregular (if so, why?), I would start by looking at the injectors. The nozzles are easy to remove and clean. I did mine in an ultrasonic cleaner after a gentle scrape with a piece of hardwood. If no improvements, start...
I would recommend buying a new hub that has the sensor already fitted. The gap between the reluctor ring (inside the hub) and sensor is critical. By all means try a replacement sensor, but be aware that the gap may require some adjustment.
A new set or rings, a cylinder hone and injector service might sort it. If you do take the head off, it's quite common to find cracks in cylinder heads and pistons on the 19J. I wouldn't worry too much about either, as long as you don't intend to...
..one thing I forgot to mention- if you do keep it, it may be worth removing the breather feed to the air cleaner and routing it to a seperate catch tank- sometimes these engines breath heavilly and deposit excessive oil in the air filter...
In my view, that is a well considered summary of the 19J from 300bhp/ton. When I bought my 90 it was fitted with its original 134k 19J and I have to say that it was a very pleasant engine. A little low on power for towing heavy loads on the road...
This is a very good question. And I'd say it is a bit of a mixed bag. I don't think such an engine will ever be worth the most. And I'd guess the majority of pre-Defenders where fitted with these over the V8. But just look at how crazy people go...
Have you been driving at extreme angles, maybe off-road? My engine is very healthy, but also does this intermittently. The 200 TDI seems to be very oil level sensitive and I avoid running it at the highest level- somewhere between the two seems...