The "old" ones had an HH60, 6HP engine. What is its problem?

The tecumseh HH 60 engine is a 6 H.P. engine. If it runs ok, keep it on the machine. It will have enough power to operate it well.
those older ones worked well at garden tilling. If you do get it running and operating, be very careful that you don't allow the engine to tip forward and hit the ground, as that tends to let the engine break off at the mounting lugs, or break the crank-case!There used to be a "bumper" arrangement that bolted onto the front, and down under the engine mount, to keep from breaking the engine casting!
Try to get a manual from MTD, as they bought out the tiller company, and are still selling parts and manuals for them.
When you get it running, carefully learn how it all works, and it will serve you well.

1971 year model build in last part 71 that 6 1/2 horsepower is mistery.

I bought the bumper guard right away. Helps keep the carb from being knocked off when you drive into the barn wall.