Getting my creation to go 1.5 to 3 MPH

[bmklawt]

I've built a miniature tractor, the size of a large lawnmower, for my father to putz around in at tractor shows, it has a John Deere LUC 2 cylinder motor coupled to a 3-speed model A transmission and axle.
This thing is way to fast, I've learned that a Model A goes 10 MPH in first gear, I need to get the speed down to 1.5 to 3 MPH and it does not need to go any faster than that.
Is there any kind of creeper gear I can install in the transmission or a different ring gear in the rear end, I've recently heard about a Warford underdrive, would that work, would I even be able to find one. My other thought would be to add a gear reducer or transfer case between the transmission and axle or just replace the axle with a gear reducer, but that seems like a ton of work.
Anyone have any thoughts or ideas on how to make this work.

Your help is greatly appreciated,