We have 3 times 34 inch Crash Tenders regularly sailing in the Leeds @ Bradford Club - two renovated vintage models and one from a modern SLEC kit.
All use one single shaft with 35mm outrunner brushless motors and to put it mildly all go well over scale speed "on the plane".
If using two shafts and motors ( as the two originals!) I would suggest using two times 28mm diameter brushless motors of around 30 mms long and 1000kv on your 3S Lipo batteries.
Two speed controller s will of course then be needed - but just one large (and shared) Lipo should save valuable weight to get easily on to the plane. This especially applies if you fit a large and weighty water pump and pipework to squirt spectators!
Good luck and hope this helps.