123S? They're not semis, they should be just fine for the green monster considering its so light weight. Though, you could go for some small AD08's if they make them that small... miniwog has been running them on his 86 for a stupid number of track days and street km's, gotta be due to the lighter weight of the car.
Actually... http://www.achillesradial.com/en/tires/racing/achilles-123s-3
Are they even available in anything under a 17?
Normal 123 perhaps?
http://www.achillesradial.com/en/tires/racing/achilles-123-2