I put a 2.25 diesel back on the road a couple of years ago, which had stood for 10 years. Performance was dire, so slow it was embarrassing. It had and still has a Fairy overdrive and 750 tyres. After a couple of weeks use the modern diesel took out the seals in the injector pump so it started leaking more fuel than it burnt. I took the pump off and had it resealed (big thumbs up to Diesel Bob who is near me). Whilst he had it in bits he found the advance part of the pump was siezed so he changed that as well. Refitted and timed it now is much improved. I think my driving style has changed as well, I anticipate hills and get momentum up. It will climb moderate hills in overdrive, it will definitely accelerate on the level in overdrive 4th gear, I tend to stick it in overdrive at anything above 30mph on the level.my
I am very slowly refreshing a 19j TD to do an engine swap but am in no hurry, I suspect my engine is a good one, the injector pump is not at the end of the slot to get enough advance into it, so the chain must not be very worn,
Hope this helps .