My 883 sportster had a 1200 kit installed when I bought it from a private owner. Do I buy spark plugs for a 883 or a 1200 ?

 

Normally the 883 and 1200 models both use the same spark plugs.

The XR 1200 series uses a different plug.

The standard XL 883/1200 have used a HD 6R12 plug since 1986-

Spark plug interchange guide-
http://www.nightrider.com/biketech/hdsparkplugs01.htm