That is not a easy job for you. Does 文曲星 support rewrite of its rom? i don't think so.
so you need a hardware which can write program into a simmilar rom of 文曲星's. and you need to build an embeded linux according to its hardware.
which means you need to know all the specifications of your 文曲星's hardware, and even you may need to write driver for them by yourself.
if you have no knowledge in embeded linux, i suggest you learn something else.
if it is possible to upgrade the system, most probably it uses flash rom. which means you can find a way to put your own OS into it. but you need a lot of information from the company to write drivers for basic input and display... (that is the most tough part) do you have any experience in writing drivers? I suggest you try out M68 first. at least there are more resource for it.
by the way, if you do not have enough money, do not every try to play with embeded systems, unless you are working in such a company.