Having cut my teeth on, and still mess with, the air-cooled cars as well as having bought/resurrected/sold a LOT of basket case rail buggies, etc. I can tell you two things I have learned:
They are super, super simple (a good thing).
They are quite often neglected (a bad thing).
In the case of the buggies, it usually goes like this:
Some guy decides to build one, he blows his whole wad on the frame. Slaps a bunch of used and questionable bits on it, has " a buddy" help him wire it up (never mind this same buddy's mobile home burnt down the year before after he installed a light switch), then they take it "wheelin'" and tear the living crap out of it unmerciful.
It breaks. Then it sits. And sits. And sits. Fixes are attempted. It breaks again. Tax return comes, they blow that on giant tires... now it breaks even faster. Sits some more. Squirrels get under the cover it hides under (these are never actually stored under a roof). They chew a few things. It sits some more.
Another buddy comes over, with a case of Coors and says "Hey, man, whatcha need ta get that there thing goin' again?". The answer is always "aw, shucks, just needs a bat'ry 'n she'll fire right up!". It never just needs a battery, and it never just fires right up. The fuel in it is now closer to shoe polish than gasoline... the tank is of course simply vented to atmosphere, no evaporative emissions collection bits on these, so all the volatiles have long since vacated the gasoline and only left behind the tar. Rain of course found its way in too, somehow. These almost always have the cheapest most garbage Autozone level open air cleaner, and the squirrel ate half the element away, and the tarp that was "new" when Reagan was in office is now barely held together and offers little to no protection from the elements. Even the squirrels have moved on (they now live in the motorhome, also abandoned and never used in the same driveway... ask the owner what it will take to get it going and, the answer inevitably will be " just needs a bat'ry".
So now you come along.... I have been in that same place. There is no "easy" way to undo the wrongs. Best bet is to start from scratch, or at least, as low of a common denominator as your budget/wife/residence/lifestyle/religion will allow. I always start with the electrics and the fuel system. The engine needs very little to run. You can isolate the entire other mess of the electrical system (it is always a mess) and just concentrate on the starter, and the power wire to the ignition coil which also will jump over to the stop solenoid and choke element on the carburetor. It needs that, and of course the ground, and that is all the electrics the engine needs to run. Charging system needs a couple more wires (you may need to polarize the generator again to make it work the best... ask me if you need to).
Probably best to either professionally clean out or just replace the fuel tank. The cheap "barrel" aluminum ones corrode up badly on the insides, if that is what it uses. Solex carbs (especially the 30s like your B engine from 1970 *should" have assuming it is all correct) is super easy to rebuild. You can often even buy those carbs brand spanking new (Brosal brand) if you want. And the mechanical fuel pump (these are almost always bad from rotten gas). Cheap and easy and widely available new. And make sure to get the correct 7mm fuel lines, these are smaller than most modern cars, as well as all the EFI air cooled Volkswagen engines.
