Airbag removal, exhaust & tires are probably the main things.
The tires are easy enough to rectify.
As for airbags, back in the mid 90's, I don't think the Civics in the similar "cheap" Honda Michelin series even had stock airbags yet. And you could bolt on a muffler on to quiet it down when not racing. They were promoted as an affordable series: "commute during the week, race on the weekend." Though I'm sure hardly anybody did that.
Good point about the cost. I wonder. Maybe they're doing it this way to ensure it's as level a playing field as possible? Everyone has exactly the same car & equipment (in theory).
View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...