Firstly,if you drive you vehicle on CNG, i strictly advise against it. Autos designed for Dedicated petrol engines don't run well on CNG.Secondly,its going to be an expensive exercise.I don't think there are any second hand slush units for the Santro/Atoz available in the market or none that i've seen (In Pindi & Peshawar).
If you're dead set on inmstalling it, you best bet would be to get a NEW (cost you and arm and a leg and then some) or a SECOND hand unit(subject to availability).Using another cars transmission will require excessive fabrication, that includes, transmission/block adaptors, drive shaft resizing, hubs and so on and so forth.