Bhai just get the installed previously on your car. It is meant to be their. Although a butterfly one will fit bolt on on you Lancer but the company has place the Piston one for a reason best known to them. It is designed for the engine hence it is to be good.
Else if u r in Karachi go to magazine lane, Shafiq mor or shershah and ask for a lancer carburetor it will cost you about 4000 with the genuine gaskits.