To get to the gearchange spring the engine has to come out for a complete strip.In theory,it is possible to remove the right hand crankcase half but,if the gear detente spring or arm [behind the primary gear,left side] move out of place [quite likely] then the clutch and primary gear will have to come off to get at it.Only one special tool is essential-a clutch removal tool.The best one is made and sold by Kevin Salt [KGS sales kevin700@btinternet.com] the cheaper ones on e bay are not up to removing a really tight clutch.He also makes a crankcase separator tool [not essential,but sometimes the crankcase halves can be a pig to separate].
A haynes manual is fairly essential if you are new to MZs,and a couple of hours study will be time well spent so you don’t finish the job [like me] with a shim or important washer left over. Good luck,Terry.