Starting a 6 volt tractor on 12 volt

I charge the battery a few times through the year. The generator does still put out some charging.

Change the bulbs to 12v, slap an alternator on it, drop in the 12v battery, connect it as negative ground and live on.

All my B's, WD, and WCs run 12v batteries on the original 6v starter.  It has been my experience that the 12v battery will throw the engine over with more authority, but you certainly won't 'hurt' it.  The Delco motor is series-wound, as the motor speed increases, it's internal inductive reactance (of the armature) conveniently limits current flow, so it cannot overload or overspeed.  What you MIGHT find, is that years of use has made the starter helix and pinion worn... having a sprightly start might reveal that age, so popping the unit out for a cleaning and inspection would be wise.

On my 'big red' JXLD (338ci inline six) driven generating plant,  Hercules spec'd the unit with 24v start.  When I went to convert it to 12v, the same part number was identified for 12v... and 8v.... and 6v.....

Would I want to start it on a 6v?  Not unless I was to use really short, really fat cables... but in looking around, I found many cases where they DID.  The starter simply didn't care- it was built to handle the current of a low-voltage start, and it limited it's current based on cranking load and speed.  That big beastie doesn't crank hard... it's just big... with a big generating head connected.