I would suggest using a standard jackshaft plate, decent clutch, and shoot for an overall ratio at least 10:1 (if you're using 26" wheel)
Maybe something like this:
Plate Only. Mount an engine to your bicycle or Mini Bike and use this Jackshaft Plate Gear Reduction. Mounts on the Lifan 2.5hp, Honda GX160 5.5, GX200 6.5 and other engines including Titan and Predator. Engine not included
gokartsusa.com
Easy to play around with gearing with those, but they have been known to chatter off the line if you're geared tall and have a cheap clutch. You can just figure out what wheel sprocket you want to run and then choose what gear you want on the plate. The Honda G100 supposedly has 3 different versions according to engine-specs.net ranging from 76CC to 98CC. All are very low compression flathead engines, but can be made to be quite torquey if you can raise the compression. The Lifan 2.5 is pretty much an updated clone of the G100K2, as far as mounting options.
Honda G100 (76 cc, 2.0 or 2.2 HP, 1.5 or 1.6 kW) engine specs: review, oil type and capacity, fuel capacity, horsepower and torque, maintenance data and tightening torque specs
www.engine-specs.net
You'll have to figure out which G100 you actually have if you want to know the actual power it makes.
Edit: It's a solid, honest engine, but isn't a powerhouse out of the box. Easy to mod, tho.