Looks like Continenatl Z129 since the oil filter is external. Does it turn?
It could have had new sleeves and pistons and if the journals were OK it could have had new bearings put in and never left the bench. I can"t tell whether the pan was off but it would have had to if bearings were checked and/or replaced All I can see is that it looks like the head was off and my guess "rebuild" could mean a valve job.
If the head is good,and it turns, it"s worth $300 if it had a valve job. The real bad issue with the Z129 is block cracking and you can"t tell that unless you pull the head.