Please...please do not buy a cub for track and trail, general duning and hardly any drag racing....you'll be very dissapointed.
However, if you're gonna build a bike that will be strictly a dune/drag racer...cub is the cheap way to go..
Keep in mind, you'll need the following:
Aftermarket head with domes cut for the motor, or a stocker cut and Oringed for it.
Better clutch
I suggest a lockout and dunable override as well....
bigger intakes and carbs
free flowing pipes (CPI, Shearer, etc.)
You already mentioned longer swinger
Stiffer suspension
longer chain, longer brake line, etc.
I've said time and time again...stock tranny and stock clutch are worthless on a 4 mil cub, even on gas....it's a waste of a good motor...
Do a search on cub, it's been covered a million times...but I'd recommend a builder that supports the site...