1981 Questions for all other cars Holden Commodore VH runnin

Australian.
I have a Holden Commodore VH from around 1981 (I think). It used to run great, but after it sitting unregistered for a year, it is now missing (it is running on 5 plugs). There is no water in the oil (or vice versa), and we changed the leads and put in all new spark plugs, checked the air leads, but to no avail. My dad thinks it might be the distributor cap. It is an old 202 blue motor.