I think that recreating a version of the stock exhaust would not be difficult or crazy expensive. My concern would be if it has been dramatically (Overlapping) re-cammed. The concern is for reversion of water back into the cylinders, and hydrolock, which breaks stuff in the rotating assy.

Do you have any idea of how it is cammed? I ask knowing that you probably do not, because whatever was done was done before you bought it.

(Note: I am not a mechanic or engine expert. But after years of being around boats and having several repowers and a few custom engines, I have picked up a couple of things.)
This is a cool animation of regular vs. performance cam and when the valves are open.

http://auto.howstuffworks.com/camshaft.htm

My intent is not to freak you out or spend your money. It is possible that the exhaust was done and the cams were never touched. It just seems reasonable to raise the issue.