if ur engine is genuine and is running fine, dont even think of replacing it with a diesel or turbo one. I also had my charade 82 on LPG for a long time, LPG at that time use to be 22/kg in summer & max 45/kg in winter. but now things are quite different, govt. is on a loot! i have sold by charade in 2005! and now i have a cultus vxr 2000 on CNG.
anyway i will suggest u to convert on CNG rather than replacing ur engine. if ur looking for economy and that u have already got experience of driving on Gas, fo gor CNG!