It's not just about form factor though, it needs to be able to do the same function or suitable replacement operations as well as the older device did.
A piece of junk that looks the same, has USB C and functions worse in every other way isn't a modern replacement for an extremely well regarded device amongst audiophiles.
Rockbox's last release looks to have been 5 years ago, so if that's non-negotiable, your best bet is probably eBay.