After having a few l34's the first place I always looked was the centre ID tag, like my first L34 8D69(4 door Torana)Z( engine prefix Z for L34, engine number would be HZ for Torana L34, D (1974) J4 (assembly plant code ) 34459 (this number matched the body tag that had all the details, moto, box, diff, colour , trim etc, also identified where it was made that tied it to the ADR plate chassis number and year code.H (LH series)This is the way I recall it, many wiser people can comment on the last part.
The stolen engine dilemma will always be around while the police play secret fn squirrell, it used to be easy when you knew any officer who could check for you when you were buying an engine, they were wanting to clear up the grubs and not ping the innocent buyers as I have heard of a few cases like above where someone buys an engine privately, then looses it when registering.
Good luck finding it, but I would guess it is long gone, but some of this stuff goes into sheds and it is only found when the grubs die, then it comes to light or pub talk gets out to those in the know