I don't recommend getting an engine swap. If you want a more fuel efficient car, you should sell cultus and buy a mehran.
Engine swaps are not always successful or recommended. Also, you will have trouble selling the car at the right price.
Mechanics recommend engine swap since they get a bargain from the new engine parts they get. Either get a new car, an engine overhaul or install CNG. Personally I would change the mechanic who suggests an engine swap. They are greedy!