If you have 6 gallons of fuel through that engine, then I would say you got your money out of it and then some.

The most I have ever seen out of an engine was 4 gallons, that was an MT-12 and I was very happy about reaching that point with it.

From my experience the connecting rod, crank, and bearing all get significant wear after several gallons of fuel.
A piston and sleeve replacement is a gamble in my opinion. The connecting rod is certainly prone to snap and the front bearing can give out.
To do it the right way you would have to replace the crank and the connecting rod as well. The cost associated with that would of course buy you a new engine.

With some engines the cost of the piston and sleeve alone will almost buy you a new engine.

With 6 gallons of fuel through it I would say grab a new engine...Of the same kind of course.
Just my opinion.

Dbow