The KW rears are adjustable Koni shocks (possibly with KW specced valving) with non linear springs. Ideally you'd want to get something with similar spring rates as the KW, or your roll resistance split will be off. Slightly harder rear springs than what KW uses should be ok too, as the standard ones are slightly too soft (imo).
The front springs on the V2 are 60N/mm (roughly 350lbs/inch), so look for rear suspension from another kit with similar front spring rate, or slightly higher.