daft q but what are these tyre sizes 120/23 0-13

Usually:
Width (mm), aspect ratio (percentage of width), wheel diameter (inches)

I'm assuming they're slicks?

Can't really help with conversion to real world measurements, but they are listed differently..

Sounds like an old X ply slick 12" wide 23" Height (eg outer diameter) for 13" rim --- type of size you might find on 1970s old Formula 2/Formula Atlantic

See http://www.avonracing.com/car_page.aspx X ply slicks 13" dia