Advice needed. A journal cap came off!

You could use the same cap over as long as it isn't cracked or messed up where the bearing goes and where it mates to the block.
Have a local shop magnaflux it and make sure it's ok to use.

If you have another cap then just check it and make sure it's the same measurements as your old one was.
Check the threds on the bolts also.
If the bolt threds are ok then the block is most likely ok.
Check the bearings on an other mains and pick a couple rods to check the bearings on.
If all is well then re-torque the mains and all the rods.
If your willing to build a new engine,why not try first fixing what you have.
I would ask a couple of shops near you why they think this would happen and if they have seen it before.
Was this a new or old engine???