Personally, i would always replace the cotters. Sure its almost $40 in OEM parts... but thats a hell of a lot cheaper than risking a dropped valve and its potential damage.
In your case, using k20a2 seats/springs/retainers... you should purchase cotters listed for those parts. If i remember correctly, k20a3/k20a2/k24a1 share the same IN/EX valves... but the seats are cut different (?).
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