Bruce, yes it is possible but it would involve nearly a complete rebuild of the total RF section, about the only original circuitry you could use is the signal conditioning and codeing you will be sending to the modulator. It would most probably cost you more (much more) to make this conversion than it would to buy a commercial unit on 6 meters even if you have the necessary test gear to assure you meet all FCC requirements. However, it you can get ahold of an old Heathkit manual for their AM unit, it would not be an impossible chore and you would have the specs on all the required parts.