I think you're undergeared and gearing should be approached as a separate issue, except as affected by tire size. For the street, optimal gearing should pair top speed with the engine spinning peak hp rpm...ideally, on paper. Reality is oftentimes a little different. Top speed potential is strictly a matter of engine horsepower, it's a reliably accurate "poor man's dyno". If the engine only makes enough hp to reach 54mph, no gearing change will increase that...without a steep, descending, grade and a tailwind. Best guess, your L125 should be able to top out close to 60mph, under ideal conditions. There's some trial & error testing/tuning in your future, if you want to be sure of dialing-in the sprocket combo. Lifans come in a number of different flavors and I don't often work with them.
You want to be within ~3% of what the engine can pull. Any taller and the bike will feel weak in top gear, when dealing with slight headwinds and even modest grades. Much shorter and the engine will feel "busy" at high cruising speeds. You may not yet realize it but, 200-300rpm can make a noticeable difference as the miles roll along. If the engine runs out of revs at 54mph, I'd estimate gearing is close to 6mph/1000rpm, in fourth gear...waaaay too low for what you want. The motor should make enough torque to pull ~7.25-7.5mph/1000rpm. From what I've seen, 110-125 Lifans reach peak power in the 7500rpm range and run out of breath in the low-mid 8000s...if the spec sheets are accurate. I'd start with 17/35; that should give you another 3.5mph. If the motor pulls it easily, the next step would be 17/34...which should increase top speed by ~5.5mph, (over what you have now) if the engine will pull it. Can't recall anyone using a 17/33 sprocket combo with an L125 but, perhaps there's a Lifan veteran reading this who knows better.
As for sustainable rpm and overheating, neither is really related to gearing. Carburetor tuning (jetting) and state of break-in are the critical parameters. Below the 1000-mile mark, expect the engine to run hot, below 500 miles...smokin' hot and those temps will peak at sustained speeds above 45-50mph. If you want to keep the engine healthy, for as long as possible, break it in properly and choose oil based on peak operating temp (which may require a long warmup, before really leaning into it), monitor temps with a dipstick oil thermometer; 20W50 gets the nod as long as oil temp exceeds 90C. And keep the oil clean. That means regular oil changes, based on total mileage and usage, along with servicing the oil spinner at least every 1K miles/yearly whichever comes first.
