What I have done in the past for coupling two shafts together, for low speed/torque, is use a piece of hose.
In your case a piece of 3/8" fuel line pressed onto the bolt threads and motor shaft (with reduction) should work. If necessary you might have to put some hose clamps on it if it slips. As mentioned by others, you will need to slow it down, otherwise the imbalance
along with difficulty of controlling wrap will be a problem.
edit: you'll need to keep the connection as short as possible, with the end of the shaft touching the bolt on the inside of the hose.