It's been my experience that almost all of the sag over time is due to
deformation of the rubber engine mounts (on the bottom, the forward ones
squash and the aft ones stretch - and vise-versa on the bottom). With the
engine mounted as it is on almost all of our aircraft (except for the large,
bed-mounted engines) the entire weight of the engine (plus some multiplier
as the entire engine is cantilevered forward of the mounts) is reacted by
the rubber mounts. Add heat, vibration, some oil and time and it's easy to
see why the front of the engine sags. And they don't have to change much to
effect a noticeable droop at the nose because of the distance from the
mounts to the cowl/spinner interface. Just a couple of degrees of droop at
the mounts will cause quite a change three feet away (or whatever) at the
nose.

I'm sure that some small amount of droop is probably caused by deformation
of the firewall - for all practical purposes, no structure can be totally
stiff, but the rubber mounts are the most likely candidate. In my opinion,
if a large part of your droop comes from firewall or the steel truss mount,
you've got other (severe) problems.

My 2 cents.

Dan Schaefer