Building muscles in leg with torn ACL

Can you work with a physical therapist? Mine was great after knee surgery, but I'd hate to recommend anything that would hurt you further.

You can do seated calf raises. Just sit on the edge of your bed or in a chair and do em just like you would if you were standing. It's not gonna offer much resistance so just really squeeze hard at the top of the movement. Also you could do standing calf raises but just be sure you are holding on to something in case your knee gives out.

ACL reconstruction sucks to recover from, I've done it myself. However I do think its necessary to push outside your comfort zone in order to regain strength and full range of motion. Get on an exercise bike ASAP and try to ride 20-30 min a day. Ankle weights are also good as you can use them to add light resistance and rebuild your quads and hamstrings. I had the benefit of athletic trainers around me for my recovery so if you are looking for more exercises or have any questions on rehab lemme know.

Yikes
Sorry to hear about your injury. Unless there were major complications, i'm unsure why you need crutches for 5-8 months.
For the first two weeks post op, the focus should be on swelling management and weight bearing as tolerated.After two weeks, the focus should be on restoring normal range of motion of the knee, ensuring that the leg can get fully straight, and close to a full bend (depending on swelling, this is typically 120-130 degrees)
The recumbent bike should be a cornerstone for your rehab. Even if you can't make a full pedal rotation, going back and forth in an arc of motion without cheating (ankle, knee hip in a straight line is the key)
Start with simple weight bearing exercise. A big part of ACL rehab is restoring your proprioception (sense of where your knee joint is in space). I would recommend by doing weight shifts side to side while standing at a counter, progressively putting more weight and more time on the affected leg, say start with 5 second reps over 5minutes. Once you get good at this you can start putting your entire body weight through the affected leg, and start moving your hands/arms around to challenge your balance, or eyes closed etc.
I would also suggest putting a rolled up towel underneath the back of your knee, put your leg out straight in front of you and try to squish the rolled up towel for 5s at a time while activating your quads and hamstrings in a co-contraction. I would also buy an exercise ball and use that to start doing mini squats (ball in small of your back to ensure good posture) ensuring good ankle, hip knee aligment, only squatting till your knees go slightly over the toes to start. Also, I would suggest you start doing gluteus medius strengthening (lying on your side, knees bent, feet together, opening like a clam).

There are many ACL protocols out there, and vary depending on the surgeon's preferences, or whether they used your patellar tendon, hamstring or allograft etc. One such example of hundreds is:
http://fowlerkennedy.com/wp-content/uploads/2013/04/ACL_Reconstruction_Protocol.pdf (I'm not specifically saying this is right for you, but it gives you an idea of what's out there)

Lastly, my main suggestion would be, on a scale of 1-10, with 1 no pain and 10 hospital pain, do not do anything that causes your pain in your knee to go beyond a 6/10 during, or for the hour after the exercise. A little pain is o.k. given the swelling and trauma you went through, but if you go past 6/10, you are likely making it more difficult for the body to heal itself.

It usually takes 6-12 months of consistent exercise to get back the muscles you lost, typically at our clinic, post ACL repairs don't start running at high levels until 5-8 months after surgery.

I have had 3 ACL reconstructions and will be having my 4th soon. I am not sure why you will be on crutches for so long on all of mine I was on crutches for 2 week tops and this next one may even be shorter. On all previous surgeries in therapy my therapist had me doing leg movements with resistance bands, wall squats, calf raises, weighted leg presses (with and without a ball between your knees), bike, treadmill, and many more. All this was as I gained range of motion and as there was little to no pain. It is by no means an easy recovery but I would imagine it would be a ton harder if you are limited on your mobility for so long. I know back when I tore my first ACL '88 (a few years before my first surgery '94) they would but the person in a full leg cast and they found that that was not good with recovery. They then went to no cast and put the patient on a flexing machine right after surgery and got the patient moving as soon as possible. Good luck on your recovery I will soon be in a similar boat as you, trying to stay active post surgery without hurting my knee again.