It'll have something like a 1% spawn rate, it might also be worth going into routes 13 and 12 that part of the map is very annoying ebcause it all runs together.
There's tonnes of gen 2 and gen 3 pokemon in the game, Ralts specifically (or at least Kirlia) is found on either route 15 or 14 (not 100% sure which as I can't remember what one I caught it on.)