I don't know what a pre-war Chinese scene would look like but I'd imagine that almost any period vehicle could be adapted to suit your needs.
I like to model in 1:72 scale have collected a few Russian vehicle kits to stand-in as "generic" civilian vehicles in dioramas. By doing a few simple modifications to the radiator grille, cabin or windshield, rear tray, running boards, etc, then giving them a civilian paint job, I think they'll pass OK. Not too many people would be familiar with Russian vehicles, so as long as they look the part, they should be fine.
Same goes for your diorama. The Tamiya Citroen Avant should work OK; they were produced in England also and considering that the three countries bordering China to the southeast were under French or British administration at the time, then it's perfectly feasible that there may have been a few Citroens getting about. Alternatively, if you want a period workhorse, Great Models have (had?) a 1:48 Gaz AA truck listed. These were a Russian-built equivalent of the Ford Model A.
Maybe a quick Google Image search on pre-war China would help??