Don't hurt to ask as someone may have already found out which it is.
Yes the ignition timing would off as it would fire on the trailing edge of the flywheel magnets instead of the leading edge. It shouldn't hurt the igniter if connected wrong on magneto system but will lead to having spark just at the wrong time and a non-start condition. And as you might think negative ground is the rule but wasn't back in the 60's or earlier. I have seen engines with points both ways. Now old cast iron Briggs are a good example of positive grounded systems.
I even have old trucks were they are positive ground and an inexperience tech it can lead to some serious damage to the electrical system.
