Are you sure the flywheel moves and the crank doesn't? It may be hard to tell unless you take the CB off. If it is slipping, just remove CB and tighten crankshaft nut.

I take it you are using a starting box. If not, make sure it's not the one-way bearing in the pull-starter (or electric starter backplate) slipping.

Endplay is probably OK.

If this is break-in time, your glow-plug may have died. They don't last long during break-in. Use RTV for sealing (best to use Copper Oxygen Sensor Safe stuff, but the regular will work). Biggest leak is usually around carb pinch bolt, and sometimes around Carb upper o-ring. Never had a backplate leak, though I hear it is common enough. If you seal the backplate, use a tiny bit of RTV smeared around the backplate where the o-ring or gasket would go. You don't want any squeezing out into the inside of the engine.