Wire from the switch to the coil can be 14 gauge or even 16 gauge or 12 gauge. All of them will work your choice. The ignition circuit is very low amps so small wire is just fine.
As for the electronic ignition it all depends on what you buy some work on 6 volts some 12 volt but me I would never install and electronic ignition on a tractor since one can replace a lot of points for what the electronic ignition cost. Shoot the D-17 I have I have run it for at least 10 years and NEVER change the points and as a matter of fact not had to do much with them since I got it. Once in a blue moon I have to file them but that is it and even having to file them has not been done in years