boygofast and others on eBay are a good source and they answer their email. Plus if there's a problem you can easily go into dispute if you have to. Some of the non-eBay dealers won't give you the time of day.
There is basically no difference in the kits. Some have decals and some don't.
As to the handle bars sick bike parts and others have a dual pull brake lever that helps somewhat. some remove the shifter and fix with 1 ratio. I bought an old single post stem mount derailllure to shift but since I put on the motor I don't shift at all.
There can be problems with mounting the sprocket on coaster-type brakes, you have to bend the arm for clearance. Stay away from aluminum bikes, too. Steel is a better choice.
This is my onw private bug about the motors. When you get one you need to tap the mounting holes for all the studs. Many times in an effort to speed the line they short tap the holes. Not enough thread engages to prevent pulling the stud right out of the block. That's the mounting holes, intake and exhaust holes and head stud holes but those take special effort because you have to remove the cylinder. A torque wrench is needed , too. The head studs will need retorqued a few time during break-in. Retorque cold, not hot. Inch pound torque wrench and tap set is $32 at Harbor Freight and more tools is always good