Cowboy, No they are probably not "hotrodded up" as that involves some complex paperwork and testing. However, they may be modified according to an existing approved mod. The exhaust system probably is not though. Crop dusters are made just for that. They are single seat special duty planes. Cessna's agtruck is one such bird. If they have been modified it is usually to go to a turboprop (jet engine with a propeller--it will have a long pointed nose) in order to get the greater power to weight ratio that it offers. These can be a bit loud, but they will only have one or at most 2 exhaust pipes 3-4 inches in diameter. The pipe can come out near the prop or back near the cowling depending on the engine. A crop duster makes alot of his noise with the prop. The low altitude and high weight demand that you move some air and that means spinning that chunk of aluminum.
Older cropdusters used the old radial engines, and while you can still get parts for the old Pratt and Wittneys it is getting harder. No american manuf still makes radials. These will have a short cowling but big and round. The exhaust from these can be just short stacks from each cylinder, but usually they have a collector ring and one or two exhaust pipes out the side or under the cowling. Here in Poland there still is a manufacturer that makes a radial. A 7 cylinder 350 hp (I think) for the STOL airplane "Wilga."
At any rate that is probably more info than you wanted.
Mike