A limited engine is bolt on stuff. So... high compression head, larger carbs, etc. We will call it a add of 5% HP. Tuned pipes use to fall under that catigory... but Factory Pipe has pulled 20% increases out of some engines... and I would start to put that into the "Mod" section.
Be careful with used impellers. I'm not saying to stay away from them... but pay a price that you won't get upset at, if you have to send it out for reconditioning.