Why mess with reductions, when you can play Menotti's excellent double-bill THE TELEPHONE and THE MEDIUM?  Scored for just 14 players in the composer's own original version.  (He does suggest you can double the strings for a more robust sound in larger theatres, but with solid players who play-out in the broader sections you don't necessarily need to).

And really you can do almost all the Handel operas with 15 players   15 would almost be luxuriously over-many for Cavalli and Monteverdi, although the early Monteverdi pieces need lots of specialist brasswind (cornetti and sackbutts, recorders etc) which don't feature in his later works.

Jonathan Dove has done some neat reductions of major works for Opera Ireland - I heard their CUNNING LITTLE VIXEN (deftly conducted by Peter Robinson) and it was a very convincing piece of work indeed, for just 15 players.

If you can summon-up around 20 players you can even do Philip Glass's AKHNATEN (as it has no fiddles at all in the score, just vla, vc & db, plus ww and brass).  There was a chamber production of it in the USA (conducted by Deirdre McLeod) which got by with just 13 players, but they had to do some small-scale rearrangement and the cellist practically never stopped playing at any point

How on earth do you move the harp part to a flute, I find myself wondering?

Good luck with the conductor!  I strongly recommend the Menotti double-bill - there's some nice writing in it, and they are neatly contrasted pieces...  THE TELEPHONE is a nice contemporary comedy (it's just a two-hander), while THE MEDIUM can be a taught thriller if it's well staged.  "Telephone" is a super piece for a Dramatic Mezzo, and it's for just five singers + one young actor (upon whom the final scene depends, so he has to be good!).  (For some unknown reason many people think Madam Flora is an "old battleaxe" part, but we know from the libretto that her daughter Monica is 17...   which puts her somewhere between 35 and 45 at most).   I am doing a MEDIUM in Moscow this Autumn, in fact, although we are doing it in Russian translation (Menotti was very keen on his works being done in the audience's language).  We did TELEPHONE last season (partnered with something else), and now we'll bring the two together.

BTW there is an arranger who advertises in "Opera" magazine in the small ads at the back, (under the name "Little Operas") who says she has a large repertoire of reductions available for hire.  Maybe worth a try?

Glad you liked the JD reduction of VIXEN, Opilec!   

Wasn't JD responsible for the Birmingham mini-Ring, too? The majority of operas written for the EOG (not just the Britten works, but works by Walton, Berkeley and Crosse, for example) have small orchestras too, and both of Martin Butler's operas use chamber forces.