There's faster single thread accelerator cards that were made for a while by a guy on the 68kMLA forums, but that was many years ago. I think Action Retro on YouTube has one that broke 2Ghz
The fastest Mac mini G4 topped out at 1.5 GHz, not 1.67. I'm the guy selling many of those Mac mini G4s upgraded with SSDs on eBay... just a hobby business. Also sell on my website at https://os9.shop where there's a FAQ where you can read more about it and why it's a good model/upgrade to buy or build yourself.
I actually bought one a year or so ago from your eBay store though, looking at my records! Have recommended to retro friends in the Seattle area, too