Last year i bought a huffy cranbrook at walmart for $40 and am near completion of my first build, 79cc also. I am not a bike guy and being my first mb i have nothing to compare it to, i think frame, rims, spokes are good, front hub is usable, i will replace rear hub soon as i don't think it's very good. However there is room for a 79cc predator. The absence of any brakes other than coaster has to be delt with, i will be adding front disc, and maybe a jack shaft brake for rear.
I was at walmart this week and it appeared that the cranbrooks currently on the floor had a new frame geometry, seat tube has a rearward bend, a few inches above bottom bracket, which should have more room for engine, seat tube on mine is straight. Tube diameters looked bigger than my cranbrook also. What you will find on the mb forums regarding box store bikes is some people love them and some hate them.
I haven't seem one, but the Columbia Klunker looks like a good platform, however I'm not sure a 79cc 4 stroke will fit, or that component qaulity is any better.
I am planning my second build, and will use either a Columbia Klunker, or Felt Bixby.
Here is a question for more experienced than i, how much $ has to be spent on a bike to get good components that you will use with confidence? Give an example of this bike.