Mac Motorcycles started as a collaboration between founder Ellis Pitt and Xenophya Design. They produce four models, each drawing inspiration from the chopper/bobber reference details. All models are powered by a Buell single cylinder, air-cooled, 492cc "Blast" engine. This type of engine is called a thumper. According to their website "it is really an event when the cylinder fires." Cool.
I must say they have a nice website. Good, detailed pictures of their models aside, you can also check out the sketches and inspiration sources. My favorite is the "Spud".