Thanks for the suggestion guys!
Here's an update on what I've tried:
I restored a backup save, but unfortunately it didn't solve my problem, since it was backed up after the relics had disappeared
I tried the debug buy mode suggested earlier, but that doesn't show the specific types of relics, just the models for them that you can place in your house, which I doubt would add to the moodlet you get for displaying a complete collection. Neat trick to know if you need to replace accidentally deleted sarcophagi and various other bits and pieces you may have lost but want to regain.
@romagia: Thank you! I just wanted to know if these relics do crop up elsewhere other than in the dungeons they belong and I believe they do, but wasn't sure.
Another possible solution for this, which might be a long way around, but you could create a new game from scratch with a 'dummy sim' get the statues you lost by sending them to do the appropriate adventures, send them home, go into edit town mode, save a copy of them and their household or just a copy of them assuming the relics are in their inventory, then load up your original save and import that 'dummy' sim into your household with the lost relics, put relics on display, delete 'dummy' sim, et voila! I dunno if this will work, but I'll give it a try and see.
Long way, but I thought hard about solutions and all is not lost! It'll just take time to get them back again
Thanks so much for all who replied and helped!