The little Honda Monkey bikes, the 50cc Z series mini bikes, are pretty popular in some countries. Some fans of the little guy really engineer very nifty modifications, like this 125cc inline 4 cylinder engine for instance. It's so small it looks like a model but it's a real engine and sounds great!
Not a lot of detail on their website but very, very cool.
Thanks for the tip, Aaron!
More photos and video below:
Link: Wirthwein Motoren