if you call PMI Europe they'll send any parts you need out to you for free. As for upgrades - there aren't many at the moment. The only things you can get at moment are new boards, feednecks (impy threaded), 15* asa's and custom triggers.
There isn't really a need for upgrades, it's lurvely stock.
www.rjskateworks.com is a custom house in America which sells Omen stuff.