I'm not giving you any info until you tell me where these bigheads are....
Seriously tho' , I don't know anything about bowfishing with a crossbow. I think you'd be much better off with an old cheap compound or recurve. I think you're going to have some difficultly finding bowfishing gear that's compatible with your crossbow, and the time it takes you to redraw/reload with your x-bow might be slow enough to drive you nuts.
Still, if it's legal... and it's what you have.... go for it.
Check over at
http://www.bowfishusa.com for more info. (but remember to come back here... because HuntingNet rules)
In Illinois there is no limit on carp (common, bighead or silver).
I have heard that the bigheads are very tasty. They do have those little intramuscular bones to watch for. Because they don't feed on the bottom, eat other fish or mollusks, I doubt they get that nice toxic build-up like common carp or catfish. I heard the state of IL was testing them just to see. (I'd like to know how toxic gar from the Illinois River are....)
I heard the bigheads were stacking up by the Peoria dam. I'm going to have to drive down there and check that out. I really want to shoot some of those things.
Good luck to you!
Christine