50 MHz Clock shifting from 5 V to 3.3 V and Bidirectional , possible?
Well, while I do not understand the need for bidirectional (for a clock of all things?) there is one component that would seem to fit the bill...

Introducing the humble transformer!

Something like an FT50-61 with maybe 10 turns tapped at 6 and cap coupled in and out, dc restore with a couple of 220 ohm resistors, would seem to fit the bill. 

Very standard fare in RF design, but possibly not that often seen on a logic board these days!