|MIG welding of heat-treatable steels is usually considered a no-no because the welds tend to be brittle and have high residual stresses unless they receive special strain relief heat treating.|
The dilemma is that used, working finish mowers cost $500. The owner wants $200 for this one and the replacement part costs $350.
I am twisting the Captain's arm. If the end of the shaft is too beat up I will pass on this offer. If the gear fits on snugly then I have a nephew who can grind off the old weld, chamfer the edges and weld them together using by preheating the parts and using high-nickel rod.
Back when I worked for a large automotive firm, the transmission division had a similar geometry and they friction welded the two parts together. They had a huge bank of flywheels and enormous, three phase electric motors. They clamped onto the shaft with a chuck, spun the flywheels up to speed and jammed the torque converter housing onto the shaft. They had something like a fifteen second cycle time.
The process ran like a champ. Eventually they brought in an electron beam welding process. The new process was never as fast, as reliable or as tolerant of dirt as the old process.
Sometimes brute force is the best answer.
If you look at problems as opportunities in work clothes, then a small shop could probably churn these out for $20 a pop and sell them for half the price of the major parts suppliers. That would be a 10X mark-up.