I think I took a bus from Beograd to Novi Sad in 2015. (I'm traveling and can't consult my notes.) If the regular trains aren't running, I'm sure there's bus service. Based on my limited observations--I bussed from Szeged (Hungary) to Subotica (Serbia) last year--the Serbian buses on such minor routes are not of the same calibre as buses used farther west.
There's some very nice Art Nouveau architecture in Oradea, though some buildings (as of 2015) looked as if they needed some restoration.
You've set yourself quite a challenge with that itinerary, time-wise. I hope you've been to Budapest before and will not need the recommended 4 to 5 days there. I'm a big fan of public transportation, but I think you will do a lot better if you drive in Serbia and Romania. I figured I covered about 30 mph on trains and buses in Romania, and I wasn't making connections.