Didn't see this mentioned yet, but make sure you are using thick heavy battery cables for your 6v system, not the small automotive 4 gauge cables found at most auto parts stores. I forget what the gauge should be, something like 00 gauge.
When it's really cold, does your TO20 turn over (even slowly) but just not catch, or is it not turning at all. In really cold weather my 20 only likes to start with the throttle only about a quarter open. If I open it to far it won't fire off as quick or at all. The coldest I have started my 6v TO20 was -18 and it fired right up. Took a while for the hydraulics to start moving with any kind of speed though!