The 2N6254 is a 100V, 15A, 150 watt transistor.  MCM Electronics
(www.mcmelectronics.com) has them in stock.

        Reverse cross referencing is tricky, as usually the 'universal subs'
choose one transistor to replace many. So, comparing the 2N6254 to the
NTE 181: original here:

They are both rated for 100V

The NTE is rated for 30 amps instead of 15A

The NTE is rated for 200 watts instead of 150 watts.

        Unfortunately, by doing a "reverse look up" you don't know if the other
device is good enough, for example, it might be rated only 12 amps.  So,
you have to carefully compare the device you are replacing (2N6254) with
the device that you want to replace it with.

        While in general, higher break down voltage, higher current and higher
dissipation ratings are OK, they also DON'T tell you about the gain of
the transistor, or the frequency response.  If you have a bunch of
output transistors in parallel, they should all be of the same type, and
in a perfect world, gain matched as well, as you want the load spread
evenly across the bank.

        The frequency response can also be a problem.  If it is too high then
the amp may have problems with oscillations.

        When using transistors that I can't curve trace, I try to use the same
lot number in the bank.  Sometimes this requires buying a few extra
transistors.