For nitrous bottles, try
http://www.detailmaster.com (also available at
http://www.plastic-models.com ) DM has a lot of really nice P/E parts.
Check out the tutorial section for Chris's (Flyonthewall) tutorial on scratchbuilding an intercooler.
The only engine packs I know of are AMT's partspacks, which are packs consisting of 50's and 60's Chevy engines. For some nice racing seats, check out
http://www.perrysresin.com/catalog.htm and click on Race parts. I'm not as good with rim manufacturers, but I beleive Fujimi makes some really nice ones. HTH
Chuck