If you buy a complete 04/05 kickstart assembly, there are several parts that will need to be updated with the 07 counterparts to add the kicker to the 06+ electric start. These parts are:
The rest of the gears and parts from the 04/05 kickstart assembly will work.
More than likely, if you buy a used 04/05 kicker assembly it's not going include the bearings for the cases. These part numbers are:
91105-KN7-671
91007-HP1-671
The actual kick lever from an 04/05 will work on 06+, but the lever will scrub the plastic a little. You will need to buy the 06+ kick lever if you want it to fit correctly in the plastic: 28300-HP1-840