[ortek_service]

Yes, Chris was trying to stick to the Acorn design as much as possible so that it looked quite similar / the same control software (that he'd updated to support further PROM types as well as his new Tesla programmer) could be used for both.

The old-favourite in 1980's projects BFY51 / 2N3053 1A transistor has now become quite-expensive (£2 each at some places! as may have been discontinued). But IIRC Chris had a good stock of these, so stayed with it, and I still have a small number in this series.
Many of the BD13x higher-current / power TO-126 types also seem to be going the same way.
However, if these transistor are only operated as a fully-saturated switch, so not much power-dissipation, then can probably use a more-common TO-92 type. The BC337 was one of my favourites, as it used to have the highest 0.5A max rating in that package. But I've recently discovered the (S)S8050 which is rated upto 1.2A by some manufacturers and I recently used one to replace a less-common 2SD467 0.7A one that had failed in a Beeb's Astec SMPSU.

I'd not come across the 74C906 open-drain CMOS-type Inverting buffer before, that appears to be obsolete and around £7 at Cricklewood (Although can get 5 for < £3 on Aliexpress). But probably some alternatives to this.
Maybe the 74LVC06A / possibly the 7406 / 74LS06 (Doesn't seem to be a 74HC06), if supply in this application is no more than 5V so doesn't require 3-15V range of the MM74C906 etc.