Those cheap motors have a working torque curve disproportionate to their supply voltage... best thing is to spend $5 and try it... but also consider...

1. You halve the playback speed, you halve the TONE... nice 400Hz Morse doesn't sound too good when played back say at 100Hz quarter speed.

2. Small DC motors running at below their optimal speed will run HOT.

Coincidentally, there I was going through a little Hamlet in deep green leafy England the other day and stopped to browse an Antiquariat that caught my eye (as if you didn't already know, girls live to shop)... There in a dusty corner was an original and boxed 12" vynil Record (you know the kind of thing people bought for entertainment before they invented CD's) - original 1960's "Teach Yourself Morse Code". As you got more proficient, you dialled up the speed on your record deck from 16rpm to 78rpm. Hands up anyone who's got a Record Deck with that speed range nowadays!