Actually.... that receiver is not probably badly cracked enough to remove it from service. That's not a highly stressed area of the receiver... I am SURE that armchair experts will disagree (awaiting flames), but... those are pretty insignificant cracks from an impact load that is not what the receiver was designed for. Unless you repeat the error, and blow the thing up again, that receiver will not see any loading in those zones again. The receiver is not generally taking much if any load there in firing, compared to the forces at the front of the receiver. If it were mine.... I'd not scrap it "just yet".
To give more detail, that area is literally as far away from the locked area of the receiver as is possible. At the moment of firing, the bolt lugs are locked into the forward most area of the receiver... this is at the extreme rear.
I would do a dye penetrant inspection of the area, and document the dimensions of the cracks, and then shoot 100 rounds and re-inspect. if the cracks stay the same, shoot another 1000 rounds and re-inspect. If the cracks stay the same... forget about it. This is exactly how we would handle a crack in an aircraft engine casing. In fact, I suggest that you take the receiver to your local airport, find the A&P (airframe and powerplant mechanic) that works there, and ask him for help in doing the dye penetrant inspection. It's cheap, easy, and accurate. Cracks in aircraft engine casings are part and parcel of them... any A&P mechanic worth his salt will be able to assist you. A crack is not absolutely a "bad thing" as long as it does not change. Good documentation of the dimensions and monitoring for changes is a smart way to proceed. If a repair is something to consider, a SMALL tap of a TIG weld on the bottom of the receiver at each crack point, and then filing off and stoning to smooth the weld area flat would likely prevent any future migration of the crack in normal shooting.
Or you could buy a new one....
By the sounds of it, this one has just about had it as far as being a shooter, and as others have opined, by the time you add an op rod, stock, and perhaps rebarrel it, you can replace it.
I'm looking for a cheap M1 receiver to build up a BM-59 project. Box it up and send it... I'm an A&P and I'll take my own advice and see how it shapes up.