I have both homebuilt and the Rickshaw mud motor. Performance wise, the homebuilt and Rickshaw are about the same. As far as pushing your boat, both will push it about 5-6 mph.
Rickshaw has a nice handle design that can be adjusted to many different configurations which is a nice thing to have. The Honda mini 4 stroke is a very nice motor. Starts right up and runs very well. Having a 3:1 reducer does help get through some stuff the homebuilt has a little harder time with.
Downside onthe Rickshaw is the cast aluminun prop. If you hit something, that prop is going to shatter. I also do not recommend the cavatation plate if it costs you extra money. All is does in weeds is get sucked into the prop which slowly grinds the prop down. I also wish the shaft was a little longer
If you are the slightest bit handy, you can build your own for around $250 max. Here is the best thread on building your own. Very long ut everything you would need to know is in there.
Mark pretty much nailed it (He owns my first one.) Yes, I'm inpressed with the design so much I bought a second one. I use one on a Fatboy. It's not a mud motor but it beats walking!
Here's a short vid where I'm running the Rickshaw on the Fatboy. That day was me (220 lbs), my gun, shell box, kayak paddle and a dozen decoys. The Fatboy weighs around 90 lbs after the redesign and added flotation. I'm running in a channel around 2-3 ft deep with little vegitation other than duck weed and grass. As I look off to the right I couldn't run in that. That would take a real mud motor.
Light weeds and grass are no problem but anything thicker and you'll be poling. I'm not sure the motor will push a Four Rivers weighing 120 lbs empty to your satisfaction. Me? I'm getting old and slow so it's not a issue just as long as I get there....;-)