https://www.thesamba.com/vw/forum/viewtopic.php?t=645213That bolt?
If so, sorry to hear. I do my best to avoid messing with that bolt for that exact reason. I have never found anything of note when pulling the strainer plate on any of my 914s and given the risk of a simple thing like that forcing you to split the engine case to repair, i dont feel its worth it to pull during regular oil changes anymore.
If the type 4 has an Achilles heel, that is it. Dumbest thing ever.
On the plus side though, you are lucky if it was the bolt itself that broke as the case also likes to crack in that area for the same reason. But to answer your question, while the motor doesn't necessarily need to be "rebuilt" it will need to come out of the car and be completely dissembled to fix it.
Sounds like you are on your way to quickly becoming experts.
Swapping a motor in a 914 is pretty easy, splitting an engine case is a bit more involved. Normally i would recommend just picking up a spare motor. It sounds like you have carbs so all your parts should swap over to any size motor you can get... but then I realized you are in Hawaii so finding a motor at a reasonable price may be easier said than done.
Not at all your sons fault though. When i was younger and inexperienced I attempted to get my oil changed at a jiffy lube one time and the guy nearly nearly did the same thing. If I had not have been watching him like a hawk he would have broke it right off.