The wrist pin bushing is brass and the fit to the wrist pin has a clearence of about .0005" to .00075". Yep! It's that close.
The big end of the connecting rod is probably one piece. In other words, there are no rod bolts on the big end. This means it is a pressed-up crank with roller bearings in the big end or crankpin as some call it. If this is the case, the case must be split, the crankshaft removed, pressed apart, the connecting rod repaired or replaced, then the whole plot cleaned 'n reassembled.
When I worked on my Mercury outboards in the early '60s, they had bushed small ends on the rods, we ran 28~32 to 1 oil 'n gas. They smoked!
Some of these Chinese engines have roller small ends, it might be cheaper just to buy one of those. I have no idea what brand or type they are. Someone here is sure to know.
If you were running cheap oil, mixing it lean or ran with a fresh air leak into the crankcase for a time or ran over heated for long periods ( like long full throttle runs on 100+F days, any of those could have helped kill the small end. Could have been a defect. The seller might have felt it going bad and sold it before it went catastrophic. Who knows? Only you and the seller.
Good luck.