I doubt you should replace _start() because it's very platform- and libc-specific. Either you write all code in assembler and so you don't need libc-specific initialization, or you should copy all _start() activity including things you aren't aware. The latter looks simply bogus.
If you agree not to replace start() but use a mechanism to run some code before main(), declare a function with __attribute__((constructor))
. This is documented GCC extension and it's actively used e.g. for static object initializing in C++. Such function can't get arguments or return a real value, nor shall it override control flow in another way. I can't catch what you mean for "transformation" so it can contradict to your intention; if so, you would have explained this more detailedly.