There's a lot of grey area.
Show up to your first race with brand new radiator hoses that don't appear in your budget, you'll get nailed. Replace yer old busted hoses with brand new ones when they break during that race, and remind the judges at the next race that you had to replace those hoses when they broke AT the last race, and they're not going to give you any grief. There's a limit to how far you can take that, though.
If you have a spare freshly built and cheaty engine mated to a freshly rebuilt transmission with a new race clutch and a lightweight flywheel and you conveniently swap it into the car when your engine spins a bearing in the 3rd lap... If you're quick with the swap, you could very well WIN that race (doubtful, but possible if it's a fast car with good drivers and pit strategy)... but it's probably not going to fly at your next BS inspection.
Bring all the spares you can think of and have room for. Once the green flag drops, keeping the car on track is the name of the game no matter what you have to put on the car to do it. Just be aware that you might have to account for parts, especially new or cheaty ones, at your next BS inspection.
Generally, if you're replacing old junkard parts with other old junkyard parts, you'll have nothing to worry about.
Lemons South 2008 - Fail, Lemons South Spring 2009 - Fail, Lemons Detroit(ish) 2008 - Fail, Lemons South Fall 2009 - Fail, Lamest Day 2009 - Fail, Miami 2010 (Chump) - 2nd!, Sebring 2010 (Chump) - Fail, Cuba 2010 - Crew Chief, Roebling 2011 (Chump) - 8th!, Sebring 2011(Chump) - 19th!