If it is done correctly...several things need to be changed and it starts getting costly. Can short cuts be made...probably. Will they be safe? Probably not.
Also, if not correctly can affect the warranty...which should still have a couple of years on it.
That being said...the following is a list of some of the basic pieces that would be needed. This does not include grommets and smaller items.
Flywheel
Starter
Starter switch
Starter/battery cable assy
Neutral safety switch assy
Wire harness assy
Rectifier (start switch picks up its power from the rectifier).
This does not include upgrading the charging circuit.
The retail price just for the parts mentioned above...$915. That does not include any labor. Part of the labor is the time to figure out exactly what will be needed.
I know of no kits or instructions available from Honda to do this.
I know it does not make any sense, but the difference between electric and pull start of a new 15HP is less than $400. Go figure.
Mike