Recreational MX/ATV Trail Design?

Yeah me.. Depending on how many mile you're talking and what kind of pay I'd possibly be interested.

I don't believe there's really any "companies" that would specialize in that alone, but there's plenty of off-road race promoters in the northeast that do basically the same thing. It kind of just comes down to what you need and who has the equipment to fit those needs (IE: Sweco dozers, or something bigger, etc)

Robby Marshall, ramtrax track builders. Give him a shout on fb.

You could check with the OHV division of the California Department of Parks and Recreation. I'd be surpised if they don't have standards or plans for the trails they improve in the off highway vehicle parks here.

There are a few MTB professional trail builders out there. Progressive Trail Designs and others. If you are just talking cross country trails, I would think that some of those guys could do it. They have experience working in the woods and the small dozers and specifically designed trail cutting equipment

Might be worth a look depending on what you want

Single track trails and Trail Dynamics are two more...

Get a topo map of area. Go walk the woods map out your course,put up arrows and ride it in.