Today is your luck day, I am snowed in and can't get out to do anything else. I have an early MK-85 and just looking at the trigger guard next to one of my Disc rifles trigger guard (aluminum alloy) they always looked like they would be interchangable. So just for you I went and removed both. The only real difference is the aluminum one has more of an opening at the rear of the trigger where it is inlet into the stock. Good news is the newer aluminum trigger guard fits right in perfectly. Trigger also functions perfectly. It may be easier to get the old one out by removing the stock from the receiver so the trigger is not in the way. the Aluminum trigger guard with the longer opening can come off and on easily with the trigger in place.
I would call Knight Customer Service and tell them yours broke, I am sure they would just send you one to replace it. I don't think any gun company wants one of their rifles out there with a broken trigger guard. If they don't have a MK-85 trigger guard, I'm sure they know the aluminum ones off the Disc Rifles will work just the same. You may just want to request the aluminum one? They will hook you up.
I have often thought about switching mine out too, but now I know it is as simple as removing two screws and replacing. No inletting neccessary!
