Unique no crank on start

I have received a wr250r from someone who does not want the hassle of fixing it.

He went to get the bike and bring it up here to sell and it started up and he got it on the trailer fine.
Upon arrival it no longer starts so he got a new starter and that didn’t fix it. Battery tested to 108 cca (100cca rating) it just gives a click then feels as if it’s locked up. Not the standard low battery constant click. Just one lock up. Oil looks good.

Tried pulling the spark plug and rolling it in gear and it would not roll, Just drag.

I pulled the engine apart and everything appears to be okay. I have not pulled one of these apart so I’m ignorant about what to check. I expected to find something obviously wrong but nothing caught my eye. Except the con rod does have minor lateral play. Is that normal? Could that cause such a catastrophic lock up?

What I find strange in that when I pull the cam chain in gear it turns over very nicely through all the gears. I’m rather lost on where to go now.
Any suggestions at all while I have the engine apart is greatly appreciated! Thank you

It's not really clear on what you have done or what the previous owner did, but try these steps in this order:

1. Place bike on a stand so rear wheel can spin freely
2. Put the bike in neutral and spin the rear wheel; does it spin freely?
3. Put the bike in gear and spin rear wheel again; does it still spin?
4. If both tests above pass, then there is no obstruction in the gear box, and you need to check valve interference in the next step:
5. Put engine back together. Remove crankshaft end cover, timing mark access screw, and spark plug; turn crankshaft counterclockwise; does it turn freely through two full circles?

To answer your question, there should no play in the connecting rod, but that play will not prevent the engine from turning. However, that play may have developed due to a more serious problem, which may be the reason the engine is not turning.