I tried a rebuild on that engine and couldn't ever get it to run right after the replaced piston/sleeve. Probably needed a new connecting rod.
As cheap as the engine is, your almost better off just buying a new one. By the time you put a new piston/sleeve/con rod ($59), another $40 will get you a new engine that doesn't have old bearings.
I ended up replacing the engine with an OS .15 cv-r(X). i'll probably never switch from OS engines. It has already lasted twice as long as the dynamite mach engine. I do the same maintainence on the OS as i did the Mach. I only got 3 gal out of the mach. I'm now on gallon #7 with the OS and it's still running really well.
I totally agree with Olds on this. Their's no point in rebuilding that engine as you can get a new one for the same or less money as a rebuild (roughly).