Somewhere I saw a project where someone made a new motherboard for a 99/4A so it would fit in a mini-itx case and tweaked the ROM so you could add an external PS/2 keyboard. After buying the board, you bought an original 99/4, yanked the chips out and used them to populate the new board.

I don't know if that really counts as "new hardware" but it was an interesting idea and was supposedly very compatible.

Or maybe I dreamed that.