Hi,
I doubt it. All the commercially available LED lights are meant for 12v input. I doubt the thought about 6v has even been thought of by the makers. Way too small a market. The lights on my 420c have been used so seldom, they are likely the original bulbs still. At least I have never changed one, and don't recall my Dad changing one, either. And, we have had that machine since 1966. But, maybe the original owner changed one between 1958 and 1966.
Not that you couldn't re-resistor 12v LED lights to work on 6v. The key thing right at the diodes themselves is current limiting, not input voltage, and that is set via a resistor in series with the diode or diode array.
Now, how to disassemble any particular unit to get at the resistors is an experiment waiting to be performed. Usually this means buying somethimg and trying to disassemble it and see what you break in an unrepairable manner and then moving onto the next make/model.
The final item is that whatever you buy will be negative ground, so you'd have to flip the diodes over in some manner to reverse their polarity.
So, something which can be done, but which I doubt anyone has.
I don't even know if there are 12v LED headlights in the form factor of round tractor lamps to begin with, but I presume there must be. I know there are for the emergency beacon spot lamps to replace incandescents, and they are the same size and shape. Now, just what would be involved with converting those to 6v I don't know, never having looked at how they were made.
Stan