I did try to search but having found anything concrete. Excuse my ignorance....is there a certain year or parts list I should be looking for (forks, upper and lower triples, brake system etc.)
I'm not sure whether you are referring to F4i or other bikes with potential USD front ends.
IF you are asking in terms of the F4i, I can say this:
The F4i forks will fit directly into the 919 triples.
F4i model run was from 2001 to 2006.
F4 is not same as F4i, only F4i had the externally adjustable damping screws.
The F4i forks ought to be reworked, including modifying the top-out spring to short and soft thus internally lengthening the fork to make it same as the 919.
The F4i triples look to have about 5 mm less offset than the 919, so would add some more trail, which is a good thing.
But no one has yet determined if the F4i triples are the same as the 919 re bearings and distance from top to bottom outer race resting faces.
Even then, the F4i top triple would need an adaptor mounted to it in order to receive handlebars, as the F4i used fork tube mounted clip-ons.
There is also the issue of fork stops, fork lock/ignition unit and 919 instrument cluster/headlamp mounting compatibility and/or adaptation needs.
IF you are asking in terms of a Honda USD conversion target, the best sounds to be the RC51 SP2, but they are very scarce nowadays.
IF you are asking in terms of the long list of Showa USD forks on other bikes (such as GSX-Rs), such forks are predominantly way too short.
Years ago, someone posted a table with 20ish or so forks and their lengths.
I thought I had it, but don't, and I spent time searching today for it on our website, but to no avail.
Not everyone cares about the resultant geometry and front end ride height of their 919 when they pursue an USD front end clip conversion.
If they intend to do any track days, they should.
Hopefully this helps you at least some.