By specialists of course.
I've seen the "stitching, but not sure if that's gonna be useful.
The center section casting on a 4 WD tractor broke in two. OOPPS!
Loaders in combination with heavy rear ballast will do that.
Seems to be a hard to get part... "furr'in make"
What you are asking is a very broad question kind of like can a piece of metal be drilled.
First you have to determine what the composition of the casting is? Some mfg such as the older caterpillar housings you can pre and post heat and cats recommended process is 7018.
Other thick sections you need to stud them and weld them.
But first thing is you have to get it cleaned best way is a oven, a lot of auto machine shops have cleaning ovens. Then determine what kind of materials you are dealing with. The materials will determine your procedure and filler materials.
You are almost sure to have some machine work on bearing bore alignment after..
You don't say what kind of box or tractor so I have no idea on cost
But as a example we repaired a gear box for a processing plant aprox 40" x 50" cracked and had 4 shafts 8 bearing bores ran right at 8k. A huge amount is in the machining after, the bearing bores are already to size so each one has to be sleeved or built up then re machined.
If you can find one I would go that route it will save you a lot of time and money.
There are several shops that specialize in just this kind of repair if you can't find one in your area let me know and I can give you a few names.
Not knowing your motorcycle guy I can't say he can or can't but there is a lot more involved than just filling a Crack.