I was waiting for my wife to check her e-mail at the Takaka (New Zealand) library when I saw this very unusual bike packer. He had fitted an engine to a mountain bike!
There was a separate chain connecting the motor to the chainring.
The fuel tank was the top tube.
The fellow claimed that the whole bike, with engine, weighed just under 20 kg. Pretty impressive. He said that the motor wasn’t that strong but still made a different on difficult terrain.
One pretty cool example of home engineering.