As far as I'm aware (as of a few years back) Green Sticker bikes are legal there and you can access up Maple Springs (or from Ortega Hwy also). I use to ride my XR/CRF50 up there all the time. And i swear riding the 50 up there was way more tiring then riding my bike up there.... not sure how that works...
Green sticker is not legal except in Wildomar OHV park. You need a plated street legal bike on Main Divide.