Firstly, I would certainly recommend the replacement parts Paolo offers - I've replaced them on both my Mamiya 7IIs with no complaints. As to how to do it, I actually went slightly against Paolo's instructions - Sorry Paolo
After unscrewing and removing the base plate [easy] I actually unscrewed the centre of the curtain crank and pulled the collar off - I'll come back to this. Once off, as my flip out levers were still ok, I actually snipped them in half with side cutters and slid both halves off the pins. Once that is done, push the pins from the centre of the round collar out - just enough that you can insert the new crank. Once inserted, just push the pins back in. Given some of the pushing and pulling to get this done I'm glad I first removed round collar entirely, but it needs to go back on in the right position. Unfortunately I didn't take note of this when I did mine but trying to close the curtain and seeing where it comes to then trying again fixed everything in a few attempts. Basically, if its in the wrong position the curtain wont close fully.