There are something like 28 causes of catastrophic failure that have been identified and without expert engine internals knowledge and a very expensive examination, you can't really be sure what was the cause.
But internal metal says probably not worthwhile for most to rebuild. Risk too great.
So source an engine. Wrecks are a good source. Wrecking yards
listed here. There are some online sources too at least one of which is already in this thread. The trick is to identify the reputable ones and pick a motor that was running when the wreck occurred. (I know my first was and is probably running in someone's car today)
On that same site is an article "
what you can do if your engine is blown" which is an accumulation of approaches I've seen over 10+ years of Boxster ownership/interest.