Can anybody tell me the make of The Fonz's
motorcycle. Thank-you

In the early episodes (where he was wearing the windbreaker) he rode a Harley Davidson Knucklehead (Knuckles were built from 1936-47) equipped with a later model wide-glide front end and Sportster gas tank.
The bike was probably a 74ci but could have been a 61ci. The most recognizable of the bikes he rode though was the silver/blue Triumph. This bike was a 1951-53 Triumph Trophy TR5. It was a 500cc pre-unit twin built primarily for the US Market (and is a pretty rare bike today). For the most part, the bike was pretty much stock throughout the series (even the high pipes on the left side were factory, as were the knobby-type tires).
In some later episodes, a more modern unit construction Triumph 650 twin was used, but the "classic" Fonzie bike will always be the TR5 Trophy.