Engine Mechanical problem
1995 Mazda MX3 4 cyl Two Wheel Drive Manual 120000 miles

My son was driving and heard a loud pop. He pulled over and tried to restart the car, it was turning over but not starting. Noticed all the oil spilled out. After getting car in our garage, I looked underneath and discovered a hole a little smaller than a golf ball in what i believe to be the crankcase cover, in between two fins. Is this something that can be replaced without pulling the whole engine? Is there a likelihood that something more serious is wrong?