There are people over on the Victorian Sweatshop forum that have painted machines and may be able to give you good tips.
Cams are always a crapshoot unless you already know for sure that the ones you buy are for your machine. Even different machines from the same manufacturer will sometimes use different styles of cams.
Like I said before, the numbers around the reverse button are for your forward stitch length, nothing to do with reverse. Whatever your forward stitch length is set at, that's what the stitches should do when you push the reverse button.
Cari