The original servo would have been a Girling mk2A which are prone to leaking and getting very difficult to source. The servo from Morris would be the closest in spec to the original. The first servo is for a V8 Rover P6 and have a very high boost ratio which you do not need. I also own a V8 Rover P6 and Amazon 133 so would have experience with these and the Rover servo wouldbe overkill. Its needed for the Rover due to the weight of the V8 cars. Before I sold my 123GT I installed the Lockheed servo from Morris minor spares into the car and it worked well. To make an easy install get this bracket
https://nordicar.com/en/product/7804...em-663713.html
If you are replacing the original Girling. It bolts directly onto the original servo bracket. Or make your own as I did