right, lets look at this in perspective......
m6 nut bouncing around in one of the chambers then white smoke.....

ok, the cylinder walls are all but bullet proof, my guess is they will be fine, the piston crown, altho ally is still thick as hell, and even the ringlands are meaty, so... i'll wager the piston is fine too [maybe a few pits, but perfectly usable]
now the bad bit......the white smoke tells me the nut 'maybe' got stuck briefly under the inlet valve and may have bent the valve, this would have distorted the valve-stem-oil-seal and let in some oil from the head.....this in turn would have made the white smoke.
best bet is to get that nut out [either plug-hole or exhaust-port with zorst-valves open] then you can turn the motor over by hand [remove other plugs first to make it easyer] whilst turning look down the plug holes to see if the inlets look paralel as they rise/fall, also check the zorst valves in the same manner [from zorst-port] you can even look at the piston crown at bdc and see for any excessive damage [you can see all this with just a pen-torch] if all looks ok, then do a comp-test....and or leak-down test..... but i fear you'll have noticed a bent valve which means head-off
best of luck bud