I found this is searching what I guess is what you have... a torque converter.
GX50 I looked up and is 4 stroke which is better for low end power, but says 2hp.
Some people have said that most torque converters are needing at least a 5hp engine to run them.
I have a 26 inch rear wheel sheave that is a cloned Chinese Whizzer type of 15 inch. My 3hp Briggs I use a jack-shaft reduction of 20:1 ratio with a centrifugal clutch. I don't have it go but 15 - 20 mph, but am exclusively using it for trail riding legal with CA OHV Reg I got. No pedals, just foot pegs.
At one time I had a smaller sheave, actually a washing machine pulley maybe 10 inches diameter and put a dual jacks-haft to get that 20:1 ratio. I don't need that extra jack-shaft any longer with the larger diameter sheave, so not to bother undoing the work, I just run a short chain connecting 2 sprockets of the same diameter.
When the washing machine pulley was worn, I would have like to find one in stamped steel like I had back in the 70's, but no luck now, everything pot metal. Some of the cloned stuff I expect is not the best. The Whizzer company, or the one company having rights to the name selling the better stuff had none in stock when I looked. Other sellers had something that were either Whizzer or in any case good reproduction of Whizzer, but I found the cost more than I wanted to pay. About 50 dollars I got a cloned version on Ebay.
I sanded the rough weld bump in the clone sheave and used my own fasteners and am satisfied with it, except under a lot of torque up hill and it can slip. Also a splash of water riding trough a stream with big enough splash and it takes a little while to dry before it grabs well again.
The torque converters that I see some of have wide belts, so they may not slip. The rear wheel with the sheave though is a weak spot using A section 4L belt. Maybe find a synchronous toothed belt and a synchronous pulley and attach to rear wheel some how. I expect dollars for synchronous stuff though.
RDD, maybe some of this may apply to your usage of the bike you build or not. Use what info you can to decide how you build based on that.