I would contact Edward Tarr in Switzerland.  He is an expert on trumpet
music.  Neruda's Concerto in E-flat was originally for the corno da cacia
but is played often on the E-flat trumpet.  Tarr is historically the
most knowledgable on composers of this era.  He also plays modern works
including those of Eugene Bozza.  He may be able to lead you to more
resources.

Hope this helps...

Bill Dishman
Gainesville, Florida