How can I fix this title problem?

You sold a bike you didn't own, which was not legal to do. Get it back, refund the money, get it in your name and then it will be legal to sell it.

Assuming you got the motorcycle as next of kin, take the title and a certified death certificate to the DMV to get clear title in your name.

What you did was illegal. The title was not in your name, you did not have the legal right to sell the bike.

You either have to get his executor to authorize a transfer application, or go back to the original owner and get them to apply for a new title. In fact, since you know their name, you can apply for a duplicate title for them, and have it sent to your address.

You will have to have his surviving spouse take a copy of his death certificate to the DMV and have it first put in their name before it can be sold. You are not legal to sell anything unless you are named in a will as the executor of his will.