body post should be an easy fix - any company's body posts will work in a pinch, just make sure you replace body, so the body doesn't end up crooked.. I know HPI has multiple kits that have body posts that'll work on the rally
the bumper, depending on how bad its damage can probably be patched. any chance you can post a pic online someplace so someone can see how badly it is?
the problem with a 'mod' is that the rally uses a kicked up front bumper, where most cars use a flat bumper - and a flat bumper won't bend to the right angles as the standard piece
sadly, rally parts are hard to come buy anymore - rest assured, you got a kick ass rally chassis. its just going to be a little more of a hunt to find the correct replacement parts. i'd never recommend (newbie or experienced) to replace composite parts with aluminum.. aluminum is nice and you get a 'bling' factor - but, in most cases, are heavier and can bend and cause other issues
Bodies By Bean | Team Belly