Thats really bad......
Not only are the bearings lubed in the oil bath, you're reduction gear box the hyrostatic motors run into also use this oil bath to keep the bearnigs and gears lubed.
If it was me, i would fill it with oil and prey it is all ok. Deal with repairs later if needed.
As you said, someone had to notice 40 odd litres of oil on the ground!
If you work in stumps ,rocks etc the 3 3/8 bolts that hold the cover over that plug can work loose, the the cover falls off, and eventually something can knock the rubber plug out. The hole is small 7/16 or 1/2” so the large volume of oil would run out rather slow, You'd think you would notice, but who knows.
I don't believe these machines have a gear reduction to worry about. But that still leaves your chains and axel bearing.
Check the end play on each axel, by pushing the wheel in and out, any movement here would be a sign of a bearing going on the axel (think the book allows .015” or so)
You might want to put a magnet into the chain case and see how much wear it collects over the next 100 or 200 hours. And watch your chains durring that time frame to see if they are stretching noticably.
Was it still kinda oily looking inside? Hopefully not all dirt or rust or metal fragments, the later being a sure sign to just park it and tear it down.