The GX50/35 are weed whacker motors and only mount from the clutch side, not from the bottom. Ideally for inframe you need a HS144f, 142f, 79cc predator, my opinion. Those are the engines most used as 4 stroke inframe mounts. Anything is possible with the right install. Just need to decide what you want. 79cc is a sweet 3hp motor, lots of threads here for guidance.
Can you get Lifan engines? Lifan is one of the higher end chinese engine manufacturers and they actually bother to meet emissions and noise regulations around the world. They also produce higher quality knock-offs as well as produce engines for major manufacturers, including Honda. The vertical 79cc enigne is engine code LF152
A Honda 50 engine doesn't come with a clutch. If you decide to go with a 30 series torque converter you won't need a clutch. With the CVT and a jackshaft you'll be able to go up any hill and still have a top level ground speed of 30 mph.
On my triple chainring shifter bike the total reduction range is 66.79~16.25\1 but I have to manually shift gears. The setup I'm telling you about the TR will be 52.8~17.6\1 and it'll automatically change as you speed up. Just give it the gas and go; no manually changing gears.
Toby, look up the specs on a Honda GXH50 and a GX50, way different motors. J above is referring to a GXH50 which comes with no clutch. GXH is probably the best 50cc 4 stroke motor you can put on a bike. Clutch is tricky though, make sure you know what works and doesn't. I think it's because of the GXH's shaft size is different, or the length.
Yes, go with the Honda. However, I provided a thread on how to attach a 30 series CVT to a small engine; you won't need a clutch. I recommend a steel frame because they're easy to weld on. A jackshaft with a 40t and 10t sprockets will also be simple to put on your bike. You'll have a unique build that nobody else currently has.