There are a couple workarounds to mate the splined areas together again. One is by welding a socket onto the bottom of the crankshaft and putting a matching hex on top of the driveshaft.
The other is by grinding a flat area on one side of the driveshaft splines and welding a matching key inside the crankshaft splined area. I don’t think you’d want to do this to a shaft with good splines though.
