Hawthorne-Sentry 558 (Colteer)

In the late 1950s my dad bought me my first rifle, a single shot 22 caliber Hawthorne-Sentry, Montgomery Ward & Co. 558. Research shows that it was manurfactured by Colt and is similar to their "Colteer". This rifle has been a great gun and I passed it down to my son and now his son. Unfortunitely, the trigger mechanism is worn out. My gunsmith is trying to find a replacement part to repair it, but has had no luck. I have searched online, but have not found this part available. Does anyone have any suggestions.