Id be starting at the top and working my way down at the top :p
Ive had collets slip out of Datto engines in the race cars (mainly after a valve bouncing eppisode due to broken gear box, missed gear or getting airbourne) and normally they made their way down to the sump with minimal damage.
Much slower moving parts in a cat engine so unless there is a problem with missig or anything then I would simply find where it cam from and replace it but the head will probably have to come off for that anyway.
The best I ever had was a circlip in the sump of a Kubota engine which was way too big for anything in the engine.
This engine got pulled down after only about 500hrs after the airfilter decided to go through the turbo and the engine.
Still no idea where the circlip came from and it's probably still sitting on my bench at my old work.:D
As far as the axle housing goes, I'd slip the centre out as a matter of course and if your replacing it anyway then theres no issue really.
Then just weld it up but like I said just give it a good clean inside and out to make sure you get a good weld on it.
Once you have a cover over it you wont even notice any difference.

