LoStOCeAnZ63

im pretty sure i need a new opti when my car is cold and it is running rich it runs alright but once it warms up it starts popping at like 3,000 rpms sometimes if i get lucky it will let me go to like 4500 rpms but anyways how hard is it to change the opti and how much are we talking? is there anything i can do to like save the one i have now becuase im broke and i just bought new wheels