Call LA County dept. of parks and rec., ask them to put you in touch of who manages the park, used to be someone based at Bonelli Park.
Once you find out if and when there is a public forum, get all the support from whoever you can, bike shop owners and personnel, local riders, IMBA, local media, anyone, just get as large as a group as you can and stress your concerns. When you get your chance to speak, be considerate, don't go on about how it just affects you( rather how it affects every user group, if they close it to bikes, they could just as easily close it to horses).
When we did it back in 1991, we had the LA Times out there, actually we were on the front page of the Sunday paper with a color photo of a trail cleanup done with other user groups. Do not give up, MC is such a great local riding area to lose. I hate to keep rattling on about how to defeat this "proposed closure" and I know I'm repeating myself, but it's a fight worth fighting, as my late father in law used to say" keep the faith"
If you need any help, let me know, I'll do what I can, although I live in Big Bear now, which is going through a trail building renaissance currently.