I'm sorry to hear that you have lost a leg. I don't know how far up the amputation was placed, or how long ago your surgery was, so in assuming the worst, here it goes.....
With your walker for stabilization, you can do curtsey squats with the remaining leg. You can also use the seated leg press at the gym if you go, as well as calf extensions, leg extensions, leg curls.
All upper body exercises can be done using body weight and circuit training machines. Crutches will help you get around better in the gym setting (I speak from experience) and they will give you one serious upper body workout and be great for a warm up as well. My triceps were toned and bulging inside of just a couple of weeks when I was laid up with a broken tibia. Although I had to modify some of the exercises, I still worked every other part of my body. I carried my protein shake and my water in a back pack that I toted with me throughout the gym. Yes it was slow going, but nonetheless, I kept on.....
MY SITUATION WAS TEMPORARY, BUT THE CONCEPT IS STILL THE SAME.
Don't forget about your abs and back muscles.
You are only limited by your imagination. And if you go to the gym, you will undoubtedly become a hero in the eyes of many. High 5's to you!!!
Kudos on your determination!!