I have a 1999 Taurus SE with DOHC 24V engine. It has just over 100,000 miles on it and there is a severe misfire on cylinders 4, 5, and 6. The engine light flashes. Compression test shows no compression on cyl 5. Rest are okay. Have been told that it needs a valve job at least. Would it be more wise to purchase a used engine or rebuild the existing one? Would like to keep the car since transmission was rebuilt just last year after 85K.