Since that sprocket looks to be pretty well mounted, I would add a jackshaft. I would put something like a 12 tooth sprocket on the wheel side of the shaft and install a hub for split sprockets on the engine side. Check that calculator that Minimichael posted. I think you should be somewhere close to 60 teeth on that end.
Remember, you MULTIPLY ratios, you do NOT ADD them.
Example: 12 tooth on clutch and 60 tooth on jackshaft would be 60:12 or 5:1
12 teeth on jackshaft and 60 teeth on the wheel is again, 5:1.
5x5=25
Your engine would need to spin 25 rotations to make the tire turn one revolution.
You can mess around with split sprockets on your jackshaft to get it the way you want it.
If that were my trike, I might use two belts from the engine to two different size pulleys so I could rig a tensioner to loosen one and tighten the other for two speeds. Have fun and be safe. Dave