All (engine) racing events have rules that specify the maximum displacement (bore, and stroke) allowed. So you can't just use a bigger engine! There are many difference between a racing engine, and a sport engine, but the most common difference would be the crankshaft, and port timing. The biggest performance gains in events where displacement is dictated by the rules, are achieved by increasing the RPM's with timing changes. This explanation is the bare bones description of a very complex subject.