It's probably thrown a rod due to oil starvation to the drive side big end. First step is to strip the crank, clear out the sludge trap. Get the crank checked for truth, good chance it will do again after a regrind. All detailed on this Forum.
If you go with the later motor, all the engine internals will interchange, but you need a nice set of plunger cases to get back on the road. '58 crank should be the later so called "Big Journal" crank, with a better choice of big end bearing shells available.
Have a look on the Forum for the various cranks and the care and attention they need, sludge traps are different on early small journal cranks. You will need to retain the existing plunger transmission, clutch etc. The crank, rods, pistons, barrel and head will all swap over. The running clearance of the timing side main bearing bush is critical for oil distribution to the crank, plenty on the Forum about this when swapping cranks between different crankcases.
Swarfy.