Depends what he paid for it. I'd roll the dice if the price was right. These things came new with left over casting junk still in the water jackets.
The way I see it, he either has to rebuild it now, or if he runs it and it fails he still has to rebuild it.
If he cleans it, rolls the dice and it turns out ok, then the gamble paid off. If it doesn't work out he was gonna be out that money anyways ripping it down and doing it all over again.
Unless there's thousands and thousands tied up in this or it's a numbers matching block, then roll the dice and get the car running and enjoy it