At a guess, going on what little I know about racing here in England (TTs mostly), I'd say for small club or open events, you can ride whatever you want, as long as it's not silly (organizing club or local/national sanctioning body probably has rules regarding bike spec). For National Champs and International events, I'd imagine you'd have to go by UCI's rules, and I wouldn't be surprised if old Steamrollers aren't UCI legal.