How do I find out other motherboards that fit my HP All-in-One - 21-2040xt CTO

[fire_bug11]

This computer is an all in one that I am interested in upgrading the entire motherboard beyond what the current one is capable. I like the AIO form factor, as it is a space saver, so I am wondering if there are any more up to date motherboards that might fit inside the AIO case, and keep it functioning as a 21 inch AIO with touch screen capabilities.

Has anyone done this before with a 21 inch AIO? If so, what did you use for the new motherboard, and how far were you able to upgrade it to?

It's currently configured with an Intel i3-4330T 3.0 ghz processor. It will go upto an i7-4785T Processor, but the MB is maxed out at 16gb, of which, I have 12gb of RAM installed.

Any advice someone could give, besides "purchase a new one" or "build your own", would be greatly appreciated.

[WAWood]

@fire_bug11 

The HP All-in-Ones are essentially giant LAPTOPS with external mice and keyboards -- and suffer from the same limitations, in that HP has their laptop keyboard custom build by outside vendors,  HP does not sell these, nor do the vendors.

While you may find a commercial Mobo that fits inside the opening, the HP mobos have special connectors that require special plugs -- and these are not found on the commercial Mobos.

IF you look online, you can usually find some for sale -- but understand all of these are USED, having been taken from existing PCs, and they may not work well, if at all.