The best way to keep the canopy from further cracking is to stop drill the crack. This means drilling a small hole on each very end of the crack. Then if you can get to the inside of the canopy I suggest repairing it on the inside by adding some fiberglass cloth and epoxy over the crack area. Be sure to sand the area before applying the cloth and epoxy. Then finish the canopy on the outside filling the stop drill holes and painting again sand before painting to give the paint a tooth to stick to.