First of all when you said it (the thing...what the He** is a thing) broke off and it's gone, does that mean JUST the bolt is gone or part of the engine mount broke off and went with the stud and nut.
If part of the engine broke, I feel it can be salvaged. I have never broke one, but went to my shop and thought how to save a broken mount. If it is the mount I'd bet it is the right side as that is the weak side. I have seen a few broken, and my feelings (they are well known on here) are that the engine wasn't mounted correctly and you didn't upgrade the hardware.
This is a rough prototype, and at midnight there are not too many stores open that I could get the material I wanted to use. So I just used what I had laying around.
In picture #1 is the right side which is the common side to break. As you can see I took a piece of flat metal and secured at the two case bolt holes. I braised a 3/4" X 3/4" 14 gage angle iron to the inside. This is evident in picture #2. In picture #3 you will see a red and blue arrow, the blue is where the mount breaks and the red is where a bolt will pass through and the head will be braised on the inside of the angle iron. Keep in mind I've never had a broken engine mount but from the looks of what I fabricated it will work. When installed the clamp will be on the outside of the angle iron...this is just so you can see it better. Trim it up..paint it, install it and I think you'd be good to go.