Weld it back on.
Clean the weld area. Grind away to clean fresh metal. Have it TIG or MIG'ed up. Grind away any welding slag and polish her up. Lots of tacks to control metal warpage. Good welder can fix this. This is a low RPM mower type engine.
Reinstall it. Rotate it in some lube checking for out of roundness with a dial indicator. All of this is way cheaper than some $500 replacement.
Take it to your local vo-tech or high school welding class. Probably get it done for free. Worth a free phone call or two.