Read post number 2.
Take the engine to someone that is good at welding aluminum.
You do not need a drain plug, just weld it closed. some Honda engines do not even have a drain plug. To change the oil use a suction into the oil filler cap.
Or go to a HF Predator engine as a last ditch effort. (which I seen quite a few do) The predators are Honda clones. Keep a heads up about shaft configuration sizes.
Also keep in mind that welding with the sump in place on the block may cause the sump gasket and the PTO seal to fail .The welder will have to be careful and not overheat the sump, weld and let it cool, then weld again.
Other option is to remove the sump then bolt it down in several places to a piece of flat steel plate to reduce chance of warpage then weld little bit at a time.
When going back with the sump use new gasket and Yamaha bond and let the Yamaha bond dry cure overnight before adding engine oil. Then install new PTO crank seal.
This might save you some big bucks on a new Honda replacement engine.