Well, most readers support epub and for the mobi format I sugguest you use the ebook manager 'Calibre' (Freeware). This programm can convert mobi format into the format you prefer.
I personally use the pocketbook touch (622) ebook reader it seem...
Have you tried to flash it with 'Flashtool'?
Have you installed the correct Kernel for your custom rom?
Another solution might be to look in the internet for 'wotan server' - here you could try to restore your phone but it will cost a little.
You could try to flash the stock rom for your phone again. Search Google if there is a stock rom ftf file and flash it with Flashtool
Another possibility is check if you have installed the correct kernel for your custom rom and install both again
I think a smartphone is too complex to build one yourself - and buying the hardware to assemble one yourself would be much more expensive (just imagine the cost for transport for every piece likely send from china)
Maybe you could start with re...