Before buying a lower unit, you may want to check the crankshaft in the power head. (if you haven't already) I bought two 40 HP big twin Evinrudes some years ago. They were late models and had been struck from behind just as you were. One engine was fine and all I had to do was replace the exhaust housing, driveshaft and lower unit. The second engine had a bent crank and was useful only for some parts.
Outside of breaking down the motor, how would you suggest I check it? She still fires up right away, idles as normal, revs fine. Only thing is when I'm running it has a bit more vibration (think the prop has a small bend in it) and the cavitation plate is cracked. After the wreck we still fished for 2 days, and a few more days before I cut the boat up. I'm not sure if the motor will last 100 years, but even if I need to rebuild the power head it will cost less then a new motor.