Well, getting back to your Classical roots has created an interesting idea. This particular fragment reminds me of what Prokoffiev did with his "Classical" symphony - it's a creative impression, rather than being the authentic article, for what that's worth. This particular idea would probably best be pursued in that light.
As a Classical Revivalist, I can say that listening carefully to what Mozart and his contemporaries did in their work and mimicking it as closely as you can without plagiarising is a good way to start.
Examine their scores and see how they accomplished effects you like. As tedious as it may sound, take a Mozart, Haydn or Beethoven symphony you admire and write a movement of it out by hand, note for note. You are bound to learn a great deal about how he accomplished his ends. This exercise is just one of many that Classicists themselves did to learn their craft.
Above all, avoid breaking rules of harmony and counterpoint. Breaking the rules they followed scrupulously is the quickest way to an ersatz, amateurish result.
In addition to a good theory text, get your hands on these books: "The Art of Counterpoint" and "The Art of Fugue" from J.J. Fux's
Gradus ad Parnassum, edited by Alfred Mann. Also, "The Great Composer As Teacher And Student," also by Alfred Mann. These will give you invaluable training and insights. Again, a little tedious, but if you're serious, "no pain, no gain."
Also, use instruments the way they would have. Avoid highly chromatic movement in the horns, for example. Horns and trumpets in the 18th Century didn't have valves, so they could only play the natural harmonics of the tube, with a few artificial alterations (look up the excellent article on Wikipedia about the horn for more information, though even that is not specific enough:
http://en.wikipedia.org/wiki/Horn_%28instrument%29 - I keep threatening to write an article on period-authentic brass writing...I may yet get around to it). That's why baroque and classical horn parts are always so spare and "waldhorn" in character.
When in doubt, do what Mozart or Haydn would have done. Don't trust your own instincts primarily - they'll lead you astray more often than not.
That's all I can think of at the moment with several ounces of Bourbon in me.