I'll throw vote number four for PowerTow.
Ours had some welds cracking, and the owner talking on the phone seemed to indicate that he knew the old design was a little flimsy there. They offered to sell a "kit" which was basically "remove your engine and transmission and drop it on this new frame" for a significant discount.
We instead found a local welder who easily patched up the thing and added structural braces.
Look for cracks where the arms meet the flat deck the motor sits atop. Note there's no diagonal bracing to keep the square arm tube from breaking off the square flat tube. The only thing holding the arm together there is the weld between the two square tube sections.
That's the stress point that will fail, if you're looking at older ones that don't have a diagonal brace or added flat plate for strength there.
Newer ones have diagonal braces across the corners of the "box" that's the flat platform the engine sits on, and added material at the "angle" part of the arms, apparently.
Our welder also didn't like that there was no fore/aft bracing underneath the flat "box", so he added a vertical piece of metal from front to rear off-set so it wouldn't get caught up in the belt. He was going to add a bunch more braces but we told him "it only gets used a couple times a week, man... don't go crazy!
).
