Given your goal of staying as original as possible, I would take the generator you have to a shop that rebuilds them (and knows what they are doing), and ask them to build a 60 amp (or 75 amp for spare capacity) 6 volt generator. Then get the best and most powerful battery you can buy (power-wise) that fits in the original case.
If you know someone who knows automotive electrical systems, consult them on the whole project so that you get your wire gauges right and have the proper grounding so the system works.
Here's one place that might be able to do the job -
http://www.auctioniowa.com/autoiowa.html (I'm sure there are others.)
