In that case, you should get plenty of cranking RPM with a good 6 volt starter, battery, and properly routed cables. Maybe you're expecting too much in the way of cranking speed? While the plugs are out, try a compression test. If it's got at least 70-80 PSI on all cylinders, it should start and run. 100+ compression pressure is better, but not really necessary. a 6.5:1 compression ratio engine would have 95 PSI of compression pressure at seal level if it was 100% efficient with no ring or valve leakage, something that's virtually impossible to accomplish. As long as all the cylinders are within 5-10% of each other on compression, the actual numbers aren't terribly important.
Jerry
