Hi Phil,
Try a wanted ad on our site’s classifieds link to track down the lower cowl. They might be a little tough to come by as that is a popular little motor. As for your decals, there is a guy on eBay who sells Martin 20 decals in vinyl, and although they don’t currently come with the logo you are looking for, I have dealt with several vendors who are willing to make them custom. The seller’s name is TK1806 — perhaps if you send him a picture if the logo, he’ll work with you.
Good luck!
JP