Not Transferred The Ownership Of The Bike

send him a legal notice through counsel to transfer the ownership in his name at earliest else be ready to face the Criminal/Civil legal action,

make a complaint to police regarding the same and also inform the authority that you have already sold your bike but he has not transferred the same you will not be responsible for any illegal activity by him involvement with this bike

File a Bike theft case in Police station , once bike is found the guy you sold will come up with his Version you sold bike to him .Based on that Take Forms signature from him . check this link for Forms you need to obtain https://www.cars24.com/blog/what-are-the-documents-required-to-sell-a-car/ and submit in your Concerned RTO that you have sold bike to also and so Take Acknowledgement from RTO your are safe meanwhile ensure Insurance is active till your Buyer Register inhis name.