BUSA has a fiberglass cowl available, and a plastic dummy engine kit. I have both, the cowl is nice, but IMO too thin; I'll layer some glass cloth inside it to beef it up before I use it. The dummy engine is okay, but I'll probably just use it as a scale guide and make my own.

Scale gear is available from Robart and from Wm Wallace. Wallace's website is www.piperj3gear.com

I'm not sure about a cockpit kit. With pictures, it wouldn't be difficult to scratch build one, not a lot of stuff in there. Some pics below, the landing gear shown is 1/5 scale, Robart in back, Wallace in front, but the 1/4 scale looks the same; then a Cub interior