"IF" both frames have the same size headtube/headset then yes the stem can be swapped over, assuming both have the Aheadset style threadless steerer on the forks.
Or that both use threaded headsets and quill type stems.
If you look in the Archive section there's a 97' Orange catalogue which will tell you some frame info eg:
Seat post diameter all seat posts 26.4 mm, shimmed to 29.8 mm... this is true of both the 97' C16-R and P7.
The only things you really need to take care with & measure are seatpost diameter, front mech clamp diameter (seat tube outer diameter, also whether the mech needs to be top pull or bottom pull depending on the cable routing on your C16-R frame) and stem/headset diameter.
Pretty much everything else will be a straight swap eg... brakes, levers, shifters, bars, saddle, rear mech etc etc...
Have you seen this thread?....
http://www.retrobike.co.uk/forum/viewto ... p;t=182168
There are plenty of Orange Guru's on Retrobike though, so I'm sure someone will know exact headset and front mech sizes for each model, though from memory I think they share the same diameters.
