Re: Will an 80cc motor fit this bike
Ditto and ditto. Bad choice, especially for a first build. That will take some custom work and fabrication even if the engine will fit in the frame. Keep looking for a better candidate bike. You'll be time and money ahead and avoid the hassles of trying to motorize that thing.
If you're not set on going with an in-frame engine you could consider a rack mount friction or chain drive. That would simplify things for you.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"