Did anyone put a servo into a 1303???

You can fit a remote servo, something like one off e.g. a bmw 2002.
One of the things on the list, just not got round to it yet.

I have already a subaru servo from a turbo one but not sure how to do it

well from what i can tell the remote system only works on single line master cylinders. so the master cyliner output goes to the servo and the servo output goes to the original T-piece that distributes the fluid to the brakes.
not sure if you have dual output master cylinder apart from how it would be mounted in a modern car ie the servo is between the brake pedal and master cylinder.
of course you will need to run a vacuum line from the engine manifolds to the servo aswell.