I was able to find a Khyber Technologies keyboard that was originally designed to be used with the MessagePad 120/130 (note this is now unavailable - I found mine of eBay).
I modified this keyboard for use with an MP2000/2100 as follows (note that to do this you will have to disassemble the keyboard and solder, so it's not for the faint of heart):
1) I removed the bracket that held the MP120/130 to the keyboard, and sealed the holes.
2) I soldered new wires to the keyboard cable, and drilled a new hole in the case to locate the cable at the opposite end of the case.
3) I drilled mounting holes on the underside of the keyboard to allow a small strip of plastic to be mounted (the gray strip seen below the company logo in the lower left picture).
4) I mounted a narrow piece of brass sheet, curved at the end, to the other end of the underside of the keyboard. A small strip of plastic was glued to the underside of the curved portion - this keeps the keyboard level in use (the brass strip seen in the lower left picture). The Newton's cover slips between the plastic strip and the curved brass piece and the keyboard case.
5) a small brass rod was bent to become the MP2100's stand. I plan to paint all exposed brass pieces black in the near future.