Consider 1,200 hours to be the equivalent of about 150,000 miles on a car. So consider the life expectancy to be more than half used up.
This obviously depends on how it was driven, what modifications and what maintenence has been done.
600 plus hours don't scare me too much, that being said, I've rebuilt engines with way less hours and five times that, so it's a crap shoot.
I think it's better for them to run than it is for them to sit.
JMO
GT