Yesterday during garden clean-up my 1973 Sears rototiller ran great for about twenty minutes and just quit. When I tried to restart it, nothing would move. After fifty years service I am afraid the motor is blown. I will attempt to tear it apart to see if there is an easy fix..that is all I am good for.

If it will not work, I need to look for another. There are plenty advertised on Marketplace, but everyone seems to think their old machines are "good as new" and are pricing them way too much. I may as well look for a new tiller. So far Champion brand looks to be inexpensive enough to consider in a tiller powerful enough to take care of my needs. Any suggestions would be helpful. I do think I need a gas powered full size tiller.

Could you possibly replace the engine?

Aren't you throwing the baby with the bath ? more likely you are just trying to raise suspense among the viewers
It would be nice to know if it is a 2 stoke or 4 stroke engine with more sources of problems.
Common causes of problems when the engine moves freely when you pull the rope : no more gas in the tank, unlikely here as the engine didn't cough a bit before stopping. The plug should be checked, if there is no spark test a new plug. If no spark a problem o ignition usually worth reparing.

Sounds like it tossed a rod or locked up.

This is what I did: took off the rope pull mechanism, WD-40ed the shaft the starter pull hooks to, put a wrench on the shaft and turned it until it felt loose, put the rope pull back on, pulled the rope and it started right up. I got lucky!

Good that you got it started! Now the question is why the thing just stopped in the middle of the job.

Maybe low on oil…just to be sure I topped up all fluids. Hopefully no internal damage.

overheated maybe
check if the fins are clean

I was hoarding a motor for years and I found an Ariens rototiller on the side of the road and the owner actually helped me put it on the back of the truck.The recoil starter screeches at me like a wife when it is running but I can just spray WD40 to stop that.
The reverse gizmo was removed so when it starts digging down to china I have to manually pull it backwards.
It is due for a new engine,probably a christmas present from harbor freight to myself.
The ariens was from the 1970's also.