The only thing that speaks to this, is the patch-box. You have done a great job on this you can tell that this one will stay, in the family. .....Does it STILL look Japanese???
There is always the option of converting this one, into a patch-box. A big factor, is how much stock you have to work with. The cap would just be the usual round plate. Either way, we are talking extra work that would mostly be cosmetic. I once had to replace a round cap that got broken and lost. ....I'd seriously consider removing that fake brass patchbox and replacing it with a real one.