This site has info on disassembly of the coaster brake.
Overhauling a Coaster Brake Bike
If that site doesn't help, do a google search on coaster brake rebuilding.
Once you have it apart, remember these words from FreeWheeler..
"The sprocket goes on first and then the brake arm - sprocket, dust cap, brake arm, bolts"
If the sprocket center hole is too small, you may need to trim the brake dust cover with a dremel, file or hacksaw. There is a post on trimming it in the "Classic Instructions" section of the forum.
I have a coaster brake bike but my motorbike has vee brakes on the rear.