JT Sprockets sell proper 428 sprockets in various sizes that fit the Honda Trail so I wouldn't bother trying to mix match a 420 sprocket from the Super Cub. The JTR269 fits the rear, you just may want to switch to lock nuts with a smaller head size so they sit flush.
i had a 13/45 on mine and it was too low, put the 14 back on and way better now, except for that dam abs light flashing A little circle of electrical tape over the light fixes that!
hondatrail125.com
The Honda Trail doesn't run out of RPM at the top of 4th so you're not likely to gain top speed by regearing alone. The motor needs to make more power to make use of the gearing.
Wayne @ The Speed Shop has swapped the semi-auto and manual transmission setups along with the crankshaft between the Grom and Trail:
The Speed Shop
www.thespeedshopmini.com
A member on this forum has either an older Grom or first generation (2021-2022 JA55) Trail 125 engine swapped into her (JA65) 2023 Trail (built by Wayne):
Hey everyone! I thought I would make my own thread to document/highlight my CT125 build progress. I purchased my 2023 CT at the end of May and have enjoyed this minimoto so much. The only regret I had post purchase was the hill climb power and even top speed even though I fully realize that, out...
hondatrail125.com
In Vietnam there's a builder who put all kinds of Honda 110/125 engines into Trail 125s built around OEM parts since the Trail 125 isn't sold there but the parts can be ordered:
Hey everyone, Im after some advice on converting a Honda scooter to a CT125. So, I'm currently living it up in Vietnam, and I've got my eye on a Honda CT125. The trouble is, they are not sold here by Honda. I could import a brand new one from Thailand, but that's gonna cost me $10,000ish USD...
hondatrail125.com
And there are some other options if you want to do a manual clutch conversion:
Just out of curiosity, has anyone done this as of yet? I still have my original grom motor (with a broken rod but still).. has anyone done this yet? I looked online but found not much.
hondatrail125.com
@m in sc currently has a regular manual hand clutch setup on his Honda Trail 125, documented in that thread. There are also conversions to just do the shifter clutch to a hand clutch and retain the centrifugal clutch so you don't have to pull the clutch in at a stop.
IMO what's more important than top speed is what speed you can pull on a mild incline, that's where the Honda Trail really struggles. There are some big bore kit builds documented on this forum where folks have improved the practical use speed of the bike so it can haul up inclines better as well as gaining some top speed on flat ground.
I'd recommend managing your expectations. If you want a faster bike you might have more fun just buying a faster bike and riding that instead of or in addition to the Trail 125. There have been quite a few people who bought the Trail 125 and sold it with low miles for various reasons, power and speed are among those reasons. It's not practical or fun for everyone and that's fine, there are plenty of other motorcycles out there.