I used to sell U45 transistors made by Central Semiconductor ... then Central Semi signed an agreement with Mouser. Shortly afterwards, Central Semi told me they were no longer interested in my business and that I had to buy their parts from Mouser. Like hell...

The BD681 has a much higher VCE saturation voltage - by about 1V (resulting in dimmer lamps).
The BD681 has a different pinout. Would need to cross two legs.
And the BD681 has nowhere near the gain that the U45's had. At nearly same load, BD681 current gain has a minimum of 750 where the U45 has a min gain of 4K. At the same load - even the TIP102 is a better choice than the BD681 (but still has wrong pinout). The gain makes a big difference considering this transistor is being driven directly from the output of a 74175 (or 74LS175). If you were driving two true lamps such as #44s then I would stick with the U45. Can probably get by with the BD681 if driving LEDs.