The SOHC valvetrain of the M103 generation engines and the SOHC valvetrain of the new twin-plug/3-vavle are totally different. The new engine has 3 valves per cyl. Two intake and one exhaust. They use the extra room to add a second spark plug. The twin-plug ignition gives a much cleaner burn than the older engine families. This means better "power-per-gallon" and less emissions. All the new engines have a trick intake that helps the torque characteristics immensely. I answered a similar question in regarding new vs old in another forum. The latest SOHC engines don't quite make the power of the previous DOHC engines but they do weigh less so there is a positive power-to-weight improvement. Give article http://www.peachparts.com/ubb/Forum5/HTML/000023.html
a look and if you have any more questions please post or email me.
Hope this helps...Lee