You can deny this accident as you have not done. Provide the letter to police station and collect the penalty amount from current owner or the police department can seize the bike against penalty.

you must address letter to RTo by regd post AD informing them of sale of your bike .

2) enclose copy of delivery note received by you and proof of payments received by you .

3) also send letter to local police station that purchaser has not transferred bike in his name yet and you apprehend bike may be used for terrorist activities .

4) you can also publish public notice in 2 local newspapers that the seller shall in no way whatsoever be responsible for any misdeeds,unlawful activities or any acts of omission or commission with the bike (state make, model,type of body,registration number, engine and chassis nos.)by the buyer or any other person who directly or indirectly uses the bike for such misdeeds,unlawful activities or any acts of omission or commission. The liability of such misdeeds,unlawful activities or any acts of omission or commission shall solely rest with the buyer of the bike ."

act as mentioned herein above 

File missing complaint. Until name not trasfer, for any accidental compensation , you will be liable. File police complaint of theft.

You send your delivery note with the buyers address and contact details to your regional RTO and local police station.  Later it will be buyers liability to get it registered

You can send a letter along with Form 29 of RTA directly to the Regional Transport authority through registered post with acknowledgement due and inform them of the fact of transfer. 
By this way the new owner name will be updated in the system and you will not be held responsible for future events.

Since you have a receipt for receiving the money for the sale consideration by welling the vehicle, you may inform RTO office about this and may mention that you do not know the current owner however you will not be liable for any accident in the past or future after having sold the vehicle.

This letter will help you to tide over the present crisis.

You do not have to worry about changing the ownership.

When you sold the vehicle to somebody and have a receipt that time itself the ownership was legally changed.

You may notify the RTO about this by sending a copy of the sales receipt which will  take care of  all such further issues.

Dear Client

if you want to safeguard yourself from this situation in future Either you have to transfer the ownership of vehicle to current owner or you can lodge a complaint with police for theft of your vehicle.

as you don't know who is the current owner of vehicle you should file a police complaint for theft of vehicle so that police will track the owner for you and you can transfer the ownership by taking the complaint back

and by this way you can also recover your money of fine from that person for taking the case of theft back