i think the bad news is gonna be that martin industries no longer exists and the company that bought them out did not carry over martins product lines and therefore parts are not available. dont quote me on this but im thinking LP and NG flue and burner designs are different and converting burners/gas valves might not be such a good idea or safe if a unit is designed for one gas and converted to another. your best bet is to get a proper fireplace for the gas you have available.
Is what you get when you didn't get what you wanted.
A positive attidude will not solve all your problems, but it will annoy enough people to make it worthwhile.