you need to look on ebay , as far as i know nobody makes lcd displays in that 4:3 format any more since years and you still will have gaps as a lcd cant follow the curve so in order to fit a lcd and if you find one you need to cut tha case of the iMac to get it straight or fill the gaps with some glue or silicon
as i had the same modding plans too ,i have a slot loader without logicboard and crt hanging around here , but i decided against and already sourced a crt that fits inside again and if you look at the new mini ....it might fit in like it is even the superdrive could line up , and as the slot loader has a opening on the back which was formerly occupied by the vga out for connecting a second monitor ..there is now space to access the ports of the mini through that door....finally found use for a mini

with a micro atx board you need to source careful all has to be as low as a new mini is , so no big heatsinks and fan assembly if you want to keep the computer in the bottom , but if you find a lcd that fits without being to big or to small , and go upstairs , then you could use the bottom space for the power supplies of the lcd and micro atx board
the lcd needs to have VISIBLE size of 29.5 widths x 22.5 height cm that is and its important as you want o use the display if you only lose the edges a little bit that would be fine ..means max visible size of the lcd can have 30 in widths and 23 height , then you might only lose a bit space in the corners and on the sides