Without knowing the specifics of the RockShox, i expect them to be modern, long travel forks (130mm and over?), so will SEVERELY impact the handling of the bike, to the point that you could consider them not usable on your bike (not to mention the front brake issue).
That, and the fact that you will need a new headset and stem to convert to AHead from threaded and quill, if you did want an upgrade over the old XCR's to something a bit more usable and quality, you may as well sell the RockShox and find some shorter travel bombers, like the ones I linked to earlier
These can also be made to run canti brakes which I am assuming you Muni Mula is running