'Upbeat and cheerfull' isn't going to get you much depth I suspect but for what it's worth Aaron Copland's El Salon Mexico, Billy the Kid and Rodeo might appeal although these are stretching the desired 'modern' somewhat. Scottish composer James MacMillan's Into the Ferment and the Berserking certainly live up to their titles. English composer Malcolm Arnold's 7th Symphony is unremittingly brilliant but maybe just too 'in yer face' confrontational to be deemed cheerfull. There is of course the unyielding and unrelenting monotony of the so-called systems music of Philip Glass and the minimalists. For me , this is music that even robots would refuse to f*ck to. That said, what's more interesting is why you want to avoid what you feel is 'sad' music. It would be revealing as to why you stipulate this criteria as something to be avoided. Art should imitate life, and sadness is an unavoidable part of life.

Some examples I came across while quickly skimming my classical music folder:

A few of these:

Sounds pretty upbeat despite the severely depressing subject matter and circumstances under which it was brought about: