My Mercury Sable engine went bad?

My mother bought a Mercury Sable 2000 from a dealer ship for ten thousand dollars with 90,000 miles now 120,000 miles, I know she got ripped off and not only that. Three weeks later the engine began to lose power and make a loud sound like pretty much a lawn mower... could this mean the spark plugs need to be changed because not all pistons work? She did an oil change maintenance every 3000 miles, obviously i need to put a new motor on there but i don't have any money and the car sounds like it is going to die eventually, but how do i minimize the sound, it also takes allot of gas... please someone give me some advice how might i make this car breathe better and run allot better then it is now... Thanks!