12j is very similar to the 10j apart from......
1984 was a 2.5-litre diesel engine. This was little more than an updated version of the existing diesel unit[32] (at this time 22 years old). The stroke was lengthened to improve torque, and an updated cylinder head was used to reduce noise and emissions. A more modern injector pump improved fuel economy and an improved glow plug system improved cold-starting performance. The most significant change was the swapping of the chain-driven camshaft for one driven by a fibre toothed belt, which also drove the repositioned injector pump.[33][34] The drive vacated by the injector pump was used to power a vacuum pump for the brake servo system. To reduce engine weight, extensive use of aluminium castings was made for the cambelt case, vacuum pump, rocker cover and other parts.
So maybe not a bad idea if your changing one anyway