The problem in this case is fortunately not with the mechanic, but the car. 100K mile is a major maintenance point for most cars; Several components are due to be changed around this time. The work that the mechanic did are not directly related to the problems that you are currently experiencing.

Since the mechanic said that the engine was OK (and the car is not sluggish), I will suggest that he clean the engine and trace where the oil in leaking from. He can also perform a cylinder-leakage test (pressurize each of the cylinders and check the adjacent cylinders, intake/exhaust manifolds, and the radiator to see where the pressure is leaking to). Based on the cost of fixing the leak compared to the price of a new engine, you can determine the way to go. I will recommend fixing the leak and changing necessary internal engine parts - I am sure Nigerian mechanics are very good in this - provided you can get quality parts.

I will personally test each engine on its merit, but on a general basis, Japanese engine should be better than American engines. The average annual commute in America is about 12K miles (app 20K Km) per year, I have seen some Japanese cars with really low odometer readings. So "IF" the engines were made to the same standards, there will be more life left in the used Japanese engine than the American one.