If you have your own lathe, and welder and cutting torch, you could cut out the center of the blade holder, turn it down, and weld up a new blade holder...
NAH, the price of one of surpluscenter's, or agrisupply's makes that obsolete... You might save the gearbox and holder, if you are of a mechanical bent...
One thing, measure the hole pattern, and ask about it, prior to ordering a new gearbox and blade holder/stumpjumper.