2-stroke engines with non-roller bearing con-rods need quite a bit of play to allow lubrication.

I reckon 5-20 thou up and down on the big end is ok, provided you dont advance the ignition above 30 degrees - you wont knock/damage it much

what the spec says i dont know, but 2 running engines had 10 and 20 thou without knocking - thats a fact

the little end should have almost no play and be a sliding fit with the piston pin, i guess 5 thou wont hurt.

LATER engines seemed to have a roller bearing con-rod - nice if you can find one - no play on these, so should run quieter?

The Big end bearing is pressed together with approx 40 tonnes. this is a specialist job and requires balancing afterwards/during. If there is roughness/rust in your big end, then you probably need a new pin, a new bearing, and worst of all a new con-rod(they pit) rods are approx £100., so if it aint broke (no noise/rust/rumbling feel) dont fix it!