yeah i just took a look and antec doesn't even show it on their site.. not even in a search result.. its probably from 2008 so 14 years old.
since windows media center pretty much died .. still working but... no support..
umm yeah all the hardware is just not there anymore.
in a universal remote it wouldn't have all the options you need but they do have Windows Media Center remotes ... I have used them in the past .. probably have one in a box some place but you can get them off ebay and amazon.. they come with a USB IR Sensor and the remote but they are optimized for Windows Media Center so... I can't tell you if they are useless with everything else .. i kinda think they are.
I haven't been there for a while but https://www.thegreenbutton.tv/ is/was the best source of information on HTPCs .. honestly .. they use to have Microsoft employees there and I have found everyone there to be very kind and helpful in the past
but I will tell you .. you are fighting a losing battle.. htpcs have honestly been replaced with TV Boxes, Android Boxes.. you can get one for $50 and set it up in a few minutes with a google account and you have access to the play store apps and Kodi, and other Media Software... and if you have $150 to spend then you have the Nvidia Shields which are much better if you subscribe to netflix or other paid services..
Personally I have gone the root of a $15 roku which lets me watch Pluto.tv and a few other free things and then i use my PC for watching youtube.
Getting some type of prepackaged TV Box that can do 4k is pretty cheap and easy today.. but if you have a setup thats working for you then you want to maintain it the best you can.
as others have said you might need to go with a wireless keyboard and mouse and you might end up powering up your htpc manually and still being able to use its other functions with a mouse and keyboard .. even an air mouse that is a USB mouse that is shaped like a remote .. it also has a keyboard on the back.. they can work with PCs if I remember right.
I applaud your dedication and wish you luck
EDIT... I don't know anything about this product but it says Windows/PC/Mac/Android.. so maybe look into a device like this I have used Air Mice before and they are marginally ok
https://www.amazon.com/Universal-Wireless-Keyboard-Connection-Projector/dp/B07WRZNW6X