Disk brake for 1972 vespa rally

hi, can i use a px or a et2 vespa forks to fit a rally 180, what do i need and how to do this convertion cheaply thankyou.  carlos algarve portugal.  

Hello,

 

You need a modified "PK" Type fork, not a PX or ET2. You have the choise for a GRIMECA type disc brake with the brake cylinder attached to the fork.
(Otherwise you have to modify the handlebar head).

Also think of converting the speedo drive shaft.

What do you mean by "cheap"?