if you want to do the occ bike, be prepared to buy some parts and make some parts.
you'll have to buy an engine mount plate(or make one) to off set the engine to the left. Without the right off set, the chain will not clear the rear tire.
you'll have to modify the rear brake caliper, make or buy an exhaust that will fit, figure out a different seat (because the occ seat is horrible).
you'll also want to shorten the clutch and throttle cables, make(or buy) a better shain tensioner, you'll want to go with a 36 tooth rear sprocket because of the small rear wheel diameter. you can get about 30 mph out of one with a 41 tooth sprocket, but that's really pushing it.
you'll need to make some kind of mount for the kit supplied gas tank to fit on the frame, or make a better looking gas tank and mount.
which ever frame you choose, do not expect to bolt the engine on in a few hours and be riding right away.
you will have to modify something along the way no matter which frame you use.
invest in some blue loc tite, and use it on every nut & bolt that has to do with mounting the engine and it's components. you'll also want to invest in some better nuts, lock nuts , lock washers and flat washers from the hardware store to replace the junk ones that come with an engine kit.
cut off the push together wire connectors, solder and heat shrink all of the wire connections.
there are also other things to consider changing for better parts (like the spark plug, spark plug wire, air filter, exhaust, etc.).
take your time, build it right, break it in right, maintain it and it will last.
throw it together quickly and something will fail quickly...I gurantee it.