Avgas is 100LL (low lead) and is sold in the paddock (at great cost), so I doubt it's illegal for proper race classes, though it would be for superbikes which are supposed to run pump fuel.

Some competitors run race fuel, which does conform to regs though I don't know how they get it over as you aren't allowed to carry fuel in cans in your vehicle on the boat. Race fuel does smell different when it burns, that's just down to the chemistry and doesn't indicate anything like nitromethane additives! Fuel sampling does go on so I don't expect there to be much (if any) cheating. The advantage of Avgas or race fuel is that since they are very consistent fuels you can tune an engine up to the limit to run on them. Pump fuel is variable so a greater margin has to be left so yuo don't destroy your engine if you get a low-grade of fuel one time.

Last point, I find it hard to believe in this day and age of computer technology and laser measurement that there isn't a better way to measure bore and stroke than pulling the head off a twin-cam multivalve engine straight after a race and putting an old pair of calipers across it.