WD40 is good at what it does but is a very poor penetrating oil. Buy a specific penetrating oil preferably with graphite, and follow the instructions on the tin. The exhaust is held onto the cylinder head by studs and nuts. The steel studs screw into the alloy head. Nuts then screw onto the studs to hold the exhaust. Try warming the engine. Don't go mad and use excessive force. If you snap the studs or strip the thread in the head you will have big problems. Soak it with penetrating oil. tap it, work the socket both ways, tighten and loosen, warm it and cool it. Be patient. When you refit use copper grease to help prevent binding. Might be an idea to see what the bike fails on before undertaking repairs.