classic motorcycle forum

I am having a "fuel" problem in the States.  I bought my sons a Honda XR 75 in 1974.  It ran with out problems for years.  This summer I restored it, but it refuses to start everytime, like it used to.  It has been suggested that the original equipment was designed for Leaded Gas and todays 87 octane just will not work.  I wonder if a different spark plug or new carburetor ports would bring back the reliability.
I welcome your suggestions and thoughts, thank you.