just cuz it is brand new exhaust doesn't mean it can't be the culprit. a quik leak test; stand exhausts up (not on engine) and fill the water passages till water comes out the exit. shut off water and look for water dripping down into exhaust ports where they bolt to the head. this test won't be a guarantee they are good, but it can be a guarantee they are bad. if the engine was always run with antifreeze, the standard rot places (intake manifold, etc) shouldn't be a problem. every 20 year old raw water intake i have ever seen has been rusty trash. i pulled one off a generator engine i bought. looked like it was new.