The very best, and most important, advice is to obtain a copy of the district or council rules.  The pack should use one of them (while the rules should be the same, they are not always).  Usually only den, and sometimes den, winners are elegible to participate in council races and districts will announce who can participate in its race.  The key person to contact for getting specific information is the District Executive (pack>>>district>>>council>>>Boy Scouts of America National Office)

There are A LOT of things that can be bought (even the body!) and done to a racer.  However, most race rules do not permit replacing nails, wheels.  Some smart units will have an open classes for anyone, sometimes separating the adults from the youth, usually siblings and 5th grade WEBELOS.  The rules can be the same up to anything goes as long as the car does not interfer with another car and must fit on the track.  It is up to the pack committee to establish the open class rules and really should stick to the district/council rules.  One suggestion is to maximize the weight since those will normally be the fastest.  Contact me at don@kentuckyvets.com for any other questions. Good luck. don