You do not need to replace the case. The 2004 case has a plug where the kickstart pokes through, pop out that plug and you are good to go. If you are taking the guts out of an 87 and putting that into the 04, than you may want to replace the locking-tab washer on the clutch. No other hard parts will be needed, but that wear parts like that locking-tab washer and the gasket for the case would be nice to have on hand. Here are the translated instructions: TW kickstart install instructions
To me it was easy, but that's a subjective subject due to different skill levels. It really is a bolt on deal.