First, welcome to OCC..........
In using the Cub Cadet Parts and Service Lookup tab at the top of the page, it reveals the correct Spark Plug is:
759-3332, Spark Plug, Resistor.
Now you can cross that to numerous brand names if you like, but one is Champion
RJ12C.
BTW: The 1605 rolled out of the Factory in 1985-86, with a Briggs & Stratton engine:
402707‐0173/0248