Hi Eugene, thanks for leaving a question. Our deepest condolences to you are your family.
In order to deregister your uncle's motorcycle, you will need to need to apply for a Grant of Probate (if there is a Will) or Grant of Letters of Administration (in Intestacy) that will authorize you to deal with the deceased's assets.
Kindly note that Grant of Probate is usually given to direct family members, and the process is more complicated if you are not directly related to the deceased.
With the authorization letter, the representative can then proceed to LTA to de-register the vehicle or transfer its ownership.