Since transistors and Mosfets have some with minimum spec's and others with maximum spec's then some will work and some will not work if ALL of them are not sensitive enough for your voltages (because you cannot order only very sensitive ones, you get whatever they have). Look in Digikey where you can select "typical" spec's for Mosfets then look on the datasheet of the most sensitive Mosfet to see if ALL of them are sensitive enough.Actually I ended up using the MGSF1N02LT1G. It seems to work. Build 10 sets and all the motors seem to be running well.
I still don't know what is a good MOSFET or BJT to use.
Audioguru, you said both the MGSF1N02LT1G and BC337 will not be fully on. But no one suggest something that will be suitable and will turn on fully. Anyone other suggestions that I can try?