Pull the backplate. Pull the head.

Then rotate the carnk slowly while you hold the piston in one position.

IMO--if you can visibly SEE slop and play in the con-rod bushing; then it's too much.

I had a K&B 48 that I got with a plane I bought from a friend. He told me the engine was a POS. Luckily, it was cheap.

The con-rod had about 1/64" play in it. Engine would run 10 minutes and deadstick. It "knocked" at idle. Didn't matter how fat I tuned the needle.