the pivot point for the rear needs to be on same plain with the engine sprocket. if it is not then the chain will become tighter and looser as the rear end swings up and down. also, you should add some tubes/sleeves the same size as the rear axle bearings so that the tube doesnt crush while tightening bolts or riding causing the axle to shift around and miss-align the chain sprockets.
@14:02 pause and take a look at where his ground is, and then take a look at where he's welding. First tip as a certified welder, know your ground placement in relation to which you are welding. Right now the current is running through the main bearings in the motor, which could potentially weld them together or create little arc strikes where the current ran through. Electricity will take the shortest route possible, so always put ground on piece that you are welding directly to. It's good practice.
