Swapping manifolds is the simplest and surest way to do it. Simply changing orifices is hit and miss and can result is a serious fir hazard.
Those grills are very prone to rusting out inside the cabinet. Keep an eye out for a curb alert (Free) one or inexpensive on the sale sites. Then grab the manifold and swap it in. Otherwise, you can probably get one from Weber but I think they are about $130.