I think they are all chinese. The gears on the bike make no difference unless you get the shift kit which costs as much as the engine itself. I have mine on a mountain bike because I didn't like the retro look. Also I wanted 29" wheels. All of these motors require some tinkering but that has been the fun. trying to get the most out of them. I'm more and more surprised each day. You'll get hooked fast if you have any mechanical ability at all.
You could surely buy your parts individually but I wouldn't do it until you have put at least one kit together first. It's fun as **** I have to tell ya.