There are many suitable motors for the 24". What kind of performance are you after?
An E-flite Park 250 (2200 kv) is a real hoot on that plane and will haul it well with floats too. Though this is on the high cost end at ~$30.
Some motors that have got my attention lately are the Turnigy 2211 motors. Available in several Kvs, something near 2000 would be my choice. Plenty of pull on 2s while swinging a 7x3.5 prop. These motors are in the $10 range and really run smooth and quiet. 16g
Though out of stock at the moment this Turnigy Easy Match Park 250 with 2200 Kv
is a strong runner as well.