Because of the rarity of the G13B Engine in Pakistan, I would recommend you to go for a G13BB Engine from a Baleno or a G16B from a Baleno 1.6GTi.
If you go for the G13BB Engine, you will not have to alter the engine mounts and basic stuff in the engine. The stock suspension is strong enuf to hold the engine as the weight difference is very small...
THe History of G13BB:
The DOHC G13BB has electronic MPFI fuel injection, generating 56–59 kW (75–79 hp) at 6,000 rpm and 104–115 N·m (77–85 ft·lbf) at 4,500 rpm, and was used in Suzuki Baleno from 1995 to 2000 and Suzuki Jimny from 1999 to 2004.
This will give you a very small and fast Cultus with high torque at low ends. CHeerz!