I went and looked around some. E1NN would go back to the early-mid 80's. I've got a 7710 with an E1NN-6015-. That's from April 85 with the block cast late 84.
Being it goes that far back in time, look at the fuel system. The engines should follow:
256 cid NA up to 10/85 will have a CAV rotary model DPA pump, and should gross about 80 hp. After this date they had a CAV Minimec inline pump at the same power spec.
268 cid NA from late 81 to late 91 will have a CAV Minimec inline pump and should gross about 90 hp.
268 T with inline Minimec pump and should gross 103 hp....
The late late versions of all three of these engines will have a CAV model DPS rotary pump.... but these pumps are fairly easily recognizable. The lines exit the end of the pump head rather than the circumference of the pump head....
The 268 NA and 268 T are easily id'd because of the turbo.
The 256 and 268 NA aren't so easy.... but the 256 has a shorter stroke, and somebody with a keen ear will pick out the 268 at torque speed. The 256 doesn't quite rattle your ear drums the same way.
If you're just looking for a used engine, and you don't care about power, ANY one of those 3 will fill the hole. They are physically identical in every way.
For that matter, any engine from '65 up to late 91 will drop right in, no problem. The later Genesis/PowerStar engines probably will too.....
The 256 was fitted to the 5610.
The 268 was fitted to the 6610 and 6710.
The 268T was fitted to the 7610 and 7710.
Various Ford TLB's used all these engines, plus some NewHolland Skid Steer's I think....
Beyond that, Valmet used them to some degree in their Forwarders.... and there were likely some other applications. HTH.