When I restored my 51, I kept/rebuilt the original 216, with the 139 tooth 6 volt flywheel. I converted everything (including the generator) to 12 volt....except the starter, kept it 6 volt. And it will spin the motor over in the extreme. You just don't want to crank it and crank it for extended periods. If everything is in time and all fuel and spark are good, then you shouldn't have to crank it for extended periods, and a 6 volt starter will last a long time, on a 12 volt system. I had a local auto electric shop convert my generator to 12 volts, and they rebuilt my 6 volt starter.
You say you started having starting trouble, as in ? We know your flywheel is correct, and the starter was/is correct, cause you say you have used it for a while. Was your 6 volt starter 'repaired', or did they sell you a new 12 volt starter? Cause if they sold you a 12 volt starter, then yep, you're gonna have issues. The bendix on a 12 volt starter is not gonna match a 6 volt system flywheel. Going to be some grinding and gnashing of teeth, so to speak.
I personally like my 6 volt starter on my 12 volt system. Works extremely well.
