Yes "fake" is perhaps not the word - usually a fake is something pretending to be something it isn't.
This bike is pretending to be anything!
The bicycle is, well, a bicycle. Perhaps 1920s-1940s, and doesn't seem to be anything special, although the bottom bracket looks a bit interesting - no cotter pins?
I don't know what the motor is, but is looks quite old. However judging from what I can see of the way it is mounted in the frame, I doubt it was originally fitted in a diamond frame like this. Do you have a photo of the other side? Motors are usually best identified by their timing side.
The petrol tank is "modern" - 1930s or later, and no manufacturer that I know of would have put a tank like this on a motorised bicycle.
Is the bike a fake? Not as it stands, but it could be if someone claims it is, say, a "1903 Wotsit". Is it "authenitic"? Probably not really - it looks like something put together for fun. Certainly the exhaust has a "fun" look about it! Built in 1948? 1963? Last year? Who can say! Maybe the owner.
Leon