I used to drive VW bugs of that vintage and I am familiar with Frazeir Park,Goman, the Grapevine, I-5, etc.
The older VW bugs had a hard time getting themselves up and over those hills. Depending on the route you take you are talking a minimum of 6% grade and could be as high as 8% grade.
A full size MX bike weights about 225 Lbs, dry, so two would be 450 Lbs. A 6 gallon gas can (full) would weigh ~40 lbs. The trailer itself would weigh about $200.
If you are hauling two bikes then I assume you have a passenger, so you are already at 50% of the typical passenger load for a VW. With all the riding gear, tools, spare parts, and the above trailer you would easily get to the 1000 Lbs mark.
That is quite a load to put on a 36 year old car that was marginal in the first place.
Adding a trailer hitch to a VW is not so easy. I strongly advise against attaching anything to the bumper, they are way too flimsy. I have seen trailer hitches added to VWs that extended up to a point where they could attach to the frame. Keep in mind that anything you put under the engine will need to be removable if/when you need to remove the engine.
As for a hitch mounted carrier: If you mounted ~300 Lbs off the rear bumper you might get your little bug to pop a wheelie! You'll certainly make it squat pretty bad.
VW bugs were very light cars. They were a great car in their day, but I would not recommend using them as a tow vehicle.
Rod
As for a hitch mounted rack: