Engine ID and overhaul?

This is a Ford 5000 1974 and it is a 4 cylinder diesel without turbocompressor. Engine crankcase has the following number stamped on it: D4NN6015?.

How do I know if it is a 256 ci engine? I am looking in 5000 parts manual and don't see this crankcase ID number.
Engine is hard to start and requires esther for a quick start. At start exhaust is smoking white for a few minutes and goes clear after. Hourmeter is indicated over 10000 hrs. I dont know real indication because tachometer cable is broken and indicator showing few hundred hours over a complete turn.

A mechanics had a look/listen to it & said: "Pretty sure that this engnine is in its original condition". He means; "Engine never overhauled". What to do with this he said is; "Change pistons, rings, connecting rod bearings and maybe cylinder head valves changed out or lapping dépends on how it seats." He talks about cylinder glazing for an engine having running too much hours at no load.

Is it possible to have an engine never overhauled with so many hours? Is it possible having cylinder glazing with a Ford diesel engine? Thought cylinder glazing is more for sleeved engine. Would you go with an engine overhaul with a tractor like this. Apart the engine hard to start, the rest of the tractor is in a pretty good condition for what i do. Ir is a though, well built and strong machine. Torque coming out of the PTO to drive a 7 feet snowblower is more than I need when snow its full bucket in melting snow.

Thanks for feedback.
Yvan

That should be the 256 engine. The "D4.." on the block number indicates 1974 vintage. The 233 was long gone by then, and the 268 was not yet produced.

Yes, it is a non-sleeved engine, and yes, it can glaze over. The decision to glaze bust vs bore should be determined once it's opened up. If there is a lot of wear in the cylinders, then it should either be bored oversize, or else sleeved and brought back down to standard.

It cannot be 10000 hrs. The counter only goes 9999.9 then starts over at 0000.0 That gen drive was a pain to keep working. After while most gave up and left it broke.

I know of many 5000 that have over 10000 hrs and one that has twice that many.