What is the correct spark plug for Briggs and stratton 3.5 classes?

J-8 works.

Instruction book says :

Models 92500 to 92599, 92900 to 92999, 110900 to 110999, and 111900 to 111999.

Champion : Short Plug : CJ-8, Long Plug : J-8, Resistor Short Plug : RCJ-8, Resistor Long Plug : RJ-8

Autolite : Short Plug : 235, Long Plug : 295, Resistor Short Plug : 245, Resistor Long Plug : 306

Robert Bosch : Short Plug : WS9E, Long Plug : ---- , Resistor Short Plug : WSR9E, Resistor Long Plug : --- . (" --- " means there is a blank shown in the list).

Spark Plug Gap : .030"" or 0.76 mm.

Ignition Point Gap : .020" or 0.51 mm.

The above was published by Briggs and Stratton of Milwaukee, Wisconsin in the USA in February 1979 and pre-dates the introduction of non-leaded petrol.