The book I'm trying to find is from approximately 1978 and I thought it was a Hardy Boys book. It was hard bound.
Inside the first page or two was an illustration of a motocross rider (dirt bike) with the front number plate having a "2" on it and the riders name is "Jock".
That's all I've got.
Looks like Hardy Boys #52 The Shattered Helmet.
There’s no mention of this character in the book description, but take a look at what you can see in Google Books. No picture, but definitely a biker character named Jock.