Been there done that but not on your model machine.
DO NOT run it or pull it until you retrieve the broken part. It will be worse if you chip it up or worse get it jammed into gears.
You can try using an electronic LCD video-cam-on-a-cable (I don't remember the name of the device but now available through HF, Ridgid, Milwaukee, Northern) and "see" if you get lucky and can fish out the part.
Based on our experience with the same event you'll wind up pulling the top of the transmission housing.
We had to remove all rear sheet metal and lay the fenders over against the rear tires. About forty bolts in all.
Going slow about 8 manhours to disassemble down to xmsn cover and about three hours to get cover off, constantly referring to book, taking pictures.
I remember we found two overly large bolts (part of ROPS maybe, not sure now) recessed down into holes. We had to scrounge up a socket that fit into the hole and it took a cheater and Kroil to get those suckers out. Something like 27 or 29-mm and had rubber suspension pads and oversize 'fender' washers we had to fish out from between two parts of the framework. Just be prepared for the odd stuff.
Speaking of Kroil, we shot everything--every fastener--twice and let it soak before we ever started. Made a world of difference.
Stand where seat was and facing rear using 3-pont arms to bump 'up' the cover and get it loose. CAUTION heavy. You'll need assistance, maybe an overhead lift to get it off and clear.
Fish out broken part. Ours came out jammed into the internal ram for the 3-point, looked like an integral part of the machine.
Reassemble with new RTV (we used namebrand 'grey' for its gasketing characteristics) on gasket mating surface after cleaning scruptuously. Prevent debris from scraping getting into the xmsn.
Get a parts book: some of the bolts are actually studs used for line-up. It'll take two or maybe three people and / or chain hoist to get everthing ready to reassemble, apply RTV, and drop entire cover over studs while maintaining lineup and integrity of sealant.
Go back with new O-rings and / or gaskets on all piping connections.
Reassemble sheet metal.
Had to use soft-face mallet to gently nudge some of the sheet metal back into position for bolt holes to line up exactly. Lube all bolts so next time is easier. Get all started before tightning any.
If any doubts at all good practice to change hydraulic fluid after all this.
Also be a good opportunity to swab out anything ("mud") that's settled into sumps--go slow and don't stir oil so you can see. Use lint-free rags or tiny air-suction hose.
Mechanically, a pretty straight forward process, just time consuming. When removing controls from fenders (if yours has them) don't kink any control lines. Slow and easy. Plastic knobs on levers may have to be replaced if they strip out.
Take pics of springs and levers of differential lock, 4-wd engagement lever, GST control, 3-point quadrant, whatever you take apart before you do and as you go along.
I hope you don't have to take it apart but would be curious how you proceed and your process and success.
In our case, we will never again use a two-piece funnel or hose-on-a-funnel or plastic funnel.
Please post back your experiences and how much time it takes.