Bix makes several different sizes, widths and heights I do believe, but even if they don't have a size that would work for your situation, there are other burners out there, or you could even make your own. I've seen several that look like aluminum blocks or round copper tubes with holes along the top that could easily be made to most any size you'd need. Beyond that you could always make a riser block with a hollow center to raise the firebox far enough to give your Bix burner clearance without permenently altering anything!
THE VERY LAST THING I WOULD EVER DO TO A COLLECTABLE ENGINE, IS MODIFY IT IN ANY PERMENENT WAY!!!