In short, what Pat is saying is that you are going to be putting a lot of unnecessary money into this car to solve some problems that you have not been able to resolve so far. It is my suspicion that the reason that these problems have not been able to be resolved, is that the people that you have tasked to do this work, don't have the necessary experience to fix the problems. This isn't saying that they are not good mechanics, or technicians, but they just don't have the know how. The closes correlation that I can give you, is a doctor that normally does internal surgeries attempting to do plastic surgery. They are both surgery, but the expertise required for both is different. You need to find an experienced old time mechanic, or at least one that understands the system as originally designed. Converting it to 12 volt is using the band aid method. If it is hard starting, then you need to find the root cause of the hard starting, now spin the motor over faster to compensate for a defective part.
Please list the various problems that you have with this car, and we can start helping resolve them, one at a time. If the car started and ran on 6 volts when new, there is no reason that it can't start and run on 6 volts today.