What spark plugs...??

The Champion 12 is equivalent to the NGK 5. Unless the engine is going to continous heavy loads [ circle track racing? ], there is no need for a colder plug. The original Honda 750 m/cycles came with Champion R6s & had points ign. You were lucky to make it around the block before they fouled...NGKs lasted a lot longer.

NGK introduced the copper centre electrode about that time. Other companies rubbished the concept, copper would melt, yada, yada...
Now, they all use copper....