A piece of flat steel, bent and hammered on a form should make a nice yoke. Just need to weld it to a shaft ( pipe? ) that will fit inside the tube on the back of the mower, and then drop on a big washer and weld a nut or the washer to the pin to keep it from falling out. Now.. for an axle, use a bolt or rod of correct diameter for the tailwheel hub.. attatch as you like.. with drill the flat ends for a hole.. weld on heave washers as reciever rings, etc.. then you can add a locknut tot he bolt.. or if a pin.. weld some washers to the end.. ugle but workable. Incedentally.. if the cheap 'deep' south.. I've seen tail wheels cut from wood boards, or rough cut lumber, and then ran plain.. or even banded with a piece of rubber.. like old tire tread, that was tacked on... center of 'wood' wheel was sleaved with apiece of pipe for a bearing/bushing...
Soundguy