Rotax max 125 dead gearbox

Hi guys. Went to rye house in the uk today, barely got to drive as my kart was being a dick and wanted to be awkward.
It would get stuck at 12k rpm so came in and had a play, got it slightly better but still not happy to rev up top.
Then i heard a noise and my chain broke, came back in sorted that.
Then hit 90 degrees C (my alfano showed 94 but on the data logging says 67, strange), so i came in and checked the cooling system. Seemed
Ok, no leaks, water in rad. Back out, same again so came in. My right leg was getting warmer than normal so i looked at my gearbox and they are fucked. Total mess in there.
I think the fucked gears was the source of all my heat.
It definitely ran with the fucked gears so my question is… what do i need to now check before i put it together with new ones? First time this has happened to me. They were plastic, i will upgrade to metal i think.
Will the bottom end be damage as the balance shaft wasnt balancing at that point. Such a shit day, managed 10 laps all day, fml.
Thanks guys