Difficult to answer. Much of this can possibly be attributed to poor timing, bad maintenance, etc. Or worse. What exactly is bad mileage?? What does the exhaust smell like? Is the check engine light on with the key on and off once the car is running? Does the car have good power, or is it bogging and hesitating? What is cold and warm idle? What is the condition/color of the oil on the dipstick? Some fixes are easy enough - maintenance, timing, fluids, tune up, etc. Others are more problematic, like a sheared crank key, etc. Are you able to run a compression test on the car??
'91 Tracker 8V, 1.6L, 5 sp, 4wd, 2dr, conv, CAMI, 'slightly' rusty
'97 Suzuki Vitara 8v, 1.6L 5 sp, 4wd, 2dr, tintop, horribly purple
'61 NSU Quickly - 150mpg