If it were me:
I would do just what you planned. It sounds like the one piston repair will last him a lot of years and you can have it done and back home quick. He doesn't need to see it gone and hear about that right when he gets home. Take pictures as you do it. When Steve is much better share the pictures with him. If he is financially able and asked to pay for it, let him pay for parts. If he is not able I would just do it. If you are afraid it would make him feel badly about it just say that you did some repairs and save the pics to show him in 5 years. I do that kind of stuff all the time for people who appreciate it or would help me out.
If he isn't able to pay you the good Lord will pay you in the end. This whole debacle is showing those around you the person you are.
I would feel differently if he was still able and was the one using it. This kind of thing is just exactly why us farm folk survive with so much against us. We have to help out and do what we can for each other. It makes us all better.