Look at the receipts and see how he made it "13:1".

Might find that's an optimistic guess and not a measured specification. If it is indeed a fact then you ought to be able to see what was done via the receipts and can undo it. SBC heads are cheap, you could probably pull the current heads, sell them and put on a set of larger chamber heads for not much out of pocket after it's all said and done.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.