Re: Will an 80cc motor fit this bike
Just from eyeballing the picture, I would say no. It might fit if you heavily angle the motor, but then you have to use a custom intake manifold to get the carb at the right angle. And the wide front downtube might give you issues too. Too many headaches right out of the gate for a new build, in my opinion.
I would just start with a bike that has a larger center triangle and avoid the headaches. An older but good quality steel mountain bike will likely be higher quality and more reliable than a newer cheap aluminum mountain bike. You can always add a front fork with shocks if you find an older bike you like that does not have one.