At low rpm, probably not.
But the engine is putting out almost 25% more gas per stroke based on simple displacement and it does rev to 9k.
So if the stock ex is good for say 6k from a 2.8, it's going to be good for maybe 4.5 k rpm on the dohc just based on simple math alone.
Will ir work, probably, wil it be ok? Only one way to find out. But if you've gone to the trouble of putting that motor in there, may as well do the whole job.
I think I built my ex from 2 1/4 inch pipe and u bends. Hardest part was finding a muffler that would fit in there.
Anything I might say is probably worth what you paid for it, so treat it accordingly!