Thanks for the various suggestions ---
The potato semi products look interesting, however, they don't appear to have a direct sales channel (it links to ebay listings which don't exist), however, there are some reps and distributors shown, so it's worth a look.
As the source can drive 50 ohm load and the sink doesn't have a specific rise/fall time specification, I may end up needing to investigate solutions that aren't based on logic gates (MMIC amplifier, impedance transformation, etc.)
Thanks for taking the time to post ideas - these have been very interesting and helpful, even if not yet leading to an obvious drop-in answer!
Dave