This is comparatively common in Europe, I wonder why there haven't been any conversion software and application notes from the US. Older US 800 MHz equipment should be as easy to convert to 33 cm as our 450 MHz phones are to the European 70 cm band (430-440 MHz).
Look for older transportable phones that use common microprocessors like Zilog Z80, Intel 8031/8051, Moto 68HC11 or Hitachi HD6303 etc. The firmware of the phone should be in an EPROM (2764, 27512 etc.) which you can erase and reprogram.
Then you should get at least the schematics for the phone. A fellow ham familiar with those processors should be able to write a program to read the keypad, drive the display and control the phone with only a help of the schematics and data sheets for the most important components. Remember that the functions used in an amateur radio transceiver are a small fraction of those needed in a cell phone.
Another ham familiar with modern (comparatively) RF technology should be able to perform the modifications needed to shift the phone to the ham band and to make a simplex radio out of a duplex phone. Don't be afraid of SMD technology, everything is just smaller so what you should do is use smaller tools and apply less heat
Of course, the more comprehensive instructions you get of the phone, the easier the job will become. Most service manuals contain good descriptions of the processor board functions, all control voltages, audio switching etc. These are comparatively easy to come by here.
Besides our pages referred to by N0PJG above, please visit the following sites to get an idea how it was done in different countries:http://home.iae.nl/users/vhaaften/nokia/index.htmlhttp://www.qsl.net/ok0ns/index_e.htmlhttp://radiomods.free.fr/
If you can't read the language, this page will convert understandable, although broken, English of many others:http://www.t-mail.com/cgi-bin/tsail