if the bike is 'lost' then you need a police report. This may cost you in baksish.
if the bike is beyond economic repair then you need to contact your carnet issuer to see what proof is needed. Good luck getting a report in Nepal... It took the RAC in UK to realise ringing a dead number for one week that the Nepal motoring organisation doesn't have an address or phone number anymore...
If you sell the bike, then it cannot be registered in Nepal unless it is under 3 years old, and then there is 300% tax for the new owner if the vehicle is under 3 years old. Not economically viable. If they don't register it then this is their choice.. bikes have been ridden for ages without a carnet... but if they have an accident etc, then I suppose the shit can hit the fan.
Maybe you could ask your carnet supplier what happens if you sell to someone.. eg. Brit sells bike to brit, and the ownership papers change. Maybe they can just re-issue the carnet in a different name, but I don't think that would help with you getting the carnet bond back.
'Life begins at the end of your comfort zone'-
UK to Asia on a DRZ400 2012-2013