I did this repair not too long ago, I don't think you have to take the brass gear off. Use a punch and hammer and gently tap the end of the shaft that is opposite the brass gear. There is a brass cap that holds the other end of the shaft. Make sure the punch is long enough to displace the entire shaft and hold all the plastic gears while the shaft is out. Its easy to do if you have a buddy or a vice to hold the speedo. If I remember correctly, the distance from the brass gear end of the shaft to the area you have to deform the shaft is 3/8". I hope this helps.
'82 300D Turbo 215k miles