None are maintenance free. Some just require less then others.
To have a motor make 15hp, plus turn 8,000 RPM and can run a lot of hours doing this is great.
The briggs WF is a factory made TAG motor. Which means "Touch-And-Go". It's a sealed motor that's not supposed to be modified. And they even come with a electric start, and pull starter.
The Blockzilla, and Star started off as jr.dragster motors. Then later became kart motors. Drag motors usually require more maintenance then kart motors. Drag motors are normally more built on the edge.