those who relate a 1970/80's 2C/D engines with modern D4D don't even deserve to ride a vehicle. 2C/D are actually detuned derivation from truck engines. As Toyota was on the brink to compete with rising oil crisis and had no time to develop from ground zero. Only advantage with 2C/D engine is capability to burn sub-standard and alternative fuels like JetA, WVO, SVO, etc etc. Such fuels may be a problem in D4D due to all electronic gunk involved in ECU.
From my test drive experience of new Hiace in Jeddah, the engine has superb smoothness, power delivery and great fuel economy with AC. Not a single excuse to ditch where fuel economy is concerned.
Regarding their availability in Pakistan, meet the pathan brother in northern Pakistan. Anything can be arranged from cross border sources, even contraband but i don't recommend that 