An MZ-34.
WAIT UP- before you go away mumbling in disgust hear this: It's NOT an MZ engine, it's a fucking monster in a small package made for light aircraft so therefore has no extra weight bullshit like motorcycle gearboxes.
They do a lot of really brilliant single and twins. Have a look at their basic model here:
http://www.compactradialengines.com/mz34.html
I've been wanting one of their two stroke twins to make a looney bike out of for ages.
http://www.compactradialengines.com/engines.html
