There some very good walk-up from the 70's to the 90's, by no means all inclusive, at this link, and they work with 21—just tested them. There are a couple of other walk-up mods that were posted over the years, but I forget just where. I'll dig through my digital 'second garage' to see if I still have them. If I find them, I'll drop a link here so you can d/l what I have.
https://forums.ootpdevelopments.com/...hlight=walk-up